My own pub, Prism & Pen, is not drowning in crap submissions, though I certainly understand what you mean. Most of our writers are engaged in about as much of a labor of love as I am. While we have had to fend off a quite small margin of obvious AI crap or otherwise thoughtless writing, that's not our usual reality.

However, amateurs writing for the love of it are not always the greatest at the mechanics. So we even though we rarely receive truly crap submissions, we (I'm not the only editor) put a lot of hours into fixing typos, reformatting to our preferred esthetics, tweaking titles, correcting spelling and grammar (while trying to respect standard punctuation, spelling and grammar rules from writers outside the United States), and trying to bring out the best in what the writer is trying to say by suggesting a clearer thesis, more evidence, better explained conclusions, etc.

It's a TON of work! Unpaid work.

And I agree with you wholeheartedly that it's a big part of whatever success Medium might experience in terms of a reputation for quality.