I mean, my native language is French where "your" is "ton" and "you're" is "tu es", yet it (rarely) happens that I mix them up in English. If I proofread I'll spot it almost every single time, but if I'm just typing my "stream of consciousness" my brain's speech-to-text module sometimes messes up.

meh, plenty of (intelligent!) native english speakers do not know all the canonical grammar rules. english contains a lot of what could be considered error correction bits, so it doesn't usually impede understanding. syntactically perfect english with weird/misused idioms (common among non-native speakers with lots of formal education) is harder to understand in my experience. I imagine this is true of most natural languages.

For what its worth as a non-native speaker I too started making this kind of errors when my English became fluent enough.