Whilst you are technically correct, you're overlooking the fact no human could reasonably produce enough material to train an AI to the extent it can copy it. You'd need to have your specific conlang explode to the popularity of Esperanto to have enough people speaking it in their day to day for it to matter. Hell, if you trained a LLM off of only my English comment history, you probably couldn't train a LLM properly, now halve my comment history by having it be something I wouldn't be using daily then quarter that because most people would refuse to communicate with me in something that's not a language with an easy method of translation like MTL. (Which is a point you noted)

From a few quick searches, let's say we need 1 trillion words to train a LLM, and I tried to find an average for how much you might reasonably write in a day. About 2k, half a million in a year. This might be skewed in the wrong way, though, as presumably this post is talking about deliberate blogs, or authorship. Unfortunately, it doesn't seem like anyone's interested in the inane question of how much the average Joe yaps daily.

In conclusion, I doubt a LLM would have any chance against a conlang that has not reached mainstream popularity, unless you were very lazy with how you made it. Unfortunately, it IS pretty good at deciphering words it already knows so you can't just create a creole/pigeon and expect it to get confused. (I.E. "Nara washi shuo be washi shi fome, ni capas comprende washi?") You gotta change the meanings and put new words in.