This comment contains a few logical fallacies.

> It states the cargo culted reasons, but not the actual truth

This dismisses existing explanations without engaging with the mentioned reasons. The following text then doesn't provide any arguments for this.

> Pronunciation is either solved by a) automatic language detection, or b) doesn't matter.

There are more possibilities than a and b. For example, it may matter for other things than pronunciation only. Also it may improve automatic detection or make automatic detection superfluous.

> If I am reading a book [...] I will pronounce it correctly, just like the screen reader will. If I see text in a language I don't recognize, I won't pronounce it correctly, and neither will the screen reader.

A generalization of your own experience to all users and systems. Screen readers aim to convey information accessibly, not mirror human ignorance.

> There's no reason that the screen reader will get it wrong, because <hungarian sentence> isn't ambiguous

This is circular reasoning. The statement is based on the assumption that automatic detection is always accurate - which is precisely what is under debate.

> If you can translate it, you already know what language it is in.

This a non sequitur. Even if someone can translate text, that doesn't mean software or search engines can automatically identify that language.

> The lang attribute adds nothing to the proces.

This absolute claim adds nothing to the logic.