I just think the entire idea of "preachiness" about exercise is wrong. When I tell people that they should start using ibuprofen/acetaminophen fixed-dose combinations (you should! They're substantially better at pain relief than anything else in the OTC market) it would be very strange for someone to jump in to announce that they have kidney problems and I have to stop preaching about things that don't work for them.

It's true that there's some people who can't exercise, and we should have sympathy for them.

As evidenced by multiple examples already in this discussion. Food, exercise, willpower, mental health, all examples where randos without a problem feel entirely comfortable preaching to the victims that they just aren't trying hard enough.