Mistakes are one thing, and most people are grateful when you point them out, privately, although, of course, there will be exceptions, especially when people don't know you and they feel you are being intrusive. But then, when you approach people on matters they conceive to be questions of style it becomes a different ballgame. Even if you are an experienced editor (ahem) and they are a relatively inexperienced writer they may insist they have the freedom to express themselves as they choose. Certainly, they are correct. And they have the freedom to be laughed at as well.
However, even there they may become "liked" and accepted in a place like Medium where issues of "quality" and standards are not that high. So, they become resistant to change.
Wanna know how they're gonna change after that?