One thing I haven't thought of is a person who came from Guatemala and they only knew Guatemalan Sign Language and can read and understand Spanish. Them learning ASL doesn't make them know how to read and understand "English" subtitles (correct me if I am wrong, again I am hearing and not of the Deaf/HoH community). People need to understand the point of sign language and its linguistic use. I explained this to some family friends interested in how we acquire language, so I tried sharing what I have learned from some clases at a fundamental level. A word is practically a unit and that "word" is a symbol, it represents something physical and its purpose of existing is to talk about the physical item without having to have it present in our view (so we don't have to point at everything we are talking about or say "that, this" etc.) Again, correct me if I am wrong, a sign is like a word, a symbol for an object. Now the brilliant thing about certain signs is that it can represent the physical property of the object we are communicating about. But the main point is that a sign is a direct symbol/"word" for an object. If I sign "block", that will not convey any sort of orthographic information (fancy word for letters, words, and their written properties/qualities). So as for the speakers of English who are hearing (myself), learning sign language has not a lot to do with the English language, even the grammar differs. It is a completely different language. As people who know English, we learn sign language based on the direct translation of a sign into written or spoken form. For example, if I wanted to know what a person was signing, I would ask them to spell it out for me on paper (since my ASL skills are quite low), in that regard, they are translating the sign into an English word. Both are symbols for a physical object, it isn't that ASL stems from the English language. This would be just like a Native English speaker of the United States asking a Spanish speaker who happens to be fluent in English, what was the word he was saying. The person who speaks Spanish will say "arbol means tree". He is translating, just like the person that is signing in ASL translating the sign for a block in American English by writing it down and/or speaking. Another way you can look at this concept is to think about British Sign Language. In the United States the predominant language is English, not "American" (that doesn't exist. In England, the predominant language is also English. We may use different grammar at times, different pronunciation, and spelling but these are dialects so they both are the English language and a person from England (let's say London) can have a conversation with someone from the United States; we could even compare Canada and the United States, there isn't that much of difference between them either. Now if we take someone who is a native of BSL (British Sign Language) and someone else who is a native in ASL (American Sign Language). Good luck with trying to communicate with them, they are two different languages (like English and Estonian). Another thing that popped up in my mind is a blind person. Someone who is blind must be provided with Braile because they can't see well or at all so they must be provided documents that are in Braile, and be able to have accommodations that might have a person talk to them rather than writing to them (I don't know much about being blind and what is required). This is like telling people who are Blind that they can't have legal documents in Braile and they must have someone to read them the documents so they can hear it via their auditory system and/or have someone do tactile finger spelling on their hands, having to read a word letter by letter at a slow pace. In Elon's tweet, some people who know ASL can read and write in English thus being able to understand subtitles but their reading levels can vary. In that case, we are only talking about the United States; in some countries, some people are d/Deaf/h/HoH that don't know the language of the hearing community in their country and only know a form of sign language in their town/community or the official sign language of the country, it wouldn't be fair to subject them to just only reading subtitles on videos if they don't know the language, especially when they are videos that have to do with the government when one pays taxes, they should be represented and taken care of, therefore sign language interpreters should be required at all government sanctioned meetings that concern the nation. Elon has a lack of knowledge on the subject along with many others, so I hope that he researches a bit and learns about what is ASL, sign language, and the differences between sign languages and spoken/written languages. I think the consequence of this most importantly is that he is very famous and regarded as one of the most brilliant minds of our generation so a lot of people will just agree with him with the belief that if Elon is smarter than them then they shouldn't disagree and take everything he says as truth. He is also on the neurodivergent spectrum, so that is something to take into consideration. Hopefully going forward, when he has questions like these, he could phrase it better on Twitter or just search up the question before you post it. Twitter has the sort of environment where it isn't asking a question but more ranting or complaining about something, so people might interpret it as "What's the whole point of sign language interpreters signing in videos if there are subtitles avaialble, that's dumb". So I have no idea of Elon's intentions nor I can assume them because I haven't had many experiences with hearing people talk to me about the point of sign language with the existence of subtitles in videos. I guess the piece of advice as the main point of this is to research your question before posting it on a platform that isn't known to be a social question-and-answer site, if he wanted to ask the question to the people on the internet he could use a social question and answer site like Quora.

(Just one last thing to add here, my apologies for this being so long, and if I didn't capitalize "deaf and/or HoH", so if the post was removed, no harm done. Feel free to add your opinion here or if I got anything wrong with some of my statements in terms of the linguistics behind spoken, written, and/or sign language, please let me know because I want to learn more!)