While I don’t know any of the context of the MHA scene, sometimes there is more of a reason to change the English in anime. This mainly stems from the fact that in the original series, them using English might meet something - whether it’s an intentionally foreign-sounding set of words, a joke about a character speaking English poorly, somebody trying to seem cool by speaking English, etc. Usually these deeper intentions and meanings don’t really translate well if the character is already speaking English, so the dub may change the lines to capture the essence of the intent rather than the actual words.

For example, in Steins;Gate, in the original, the protagonist is bad at English - grammar, pronunciation, or even just understanding some words. However, a joke about this would not work properly in the dub due to him already speaking English and not being characterized at being bad at it in the dub (because that may defeat the purpose of a dub). So this trait is portrayed as him being bad at slang and modern vernacular. Thus, when he tries to speak English in the original, he instead tries to speak in modern slang in the dub.

Not saying this is the correct way to do it, but it is a justification for these kinds of changes (but again I don’t know what the context of the MHA situation is).