You're correct that it's not a magic bullet for everyone, but it's not some sliding scale where exercise is correlated to only works for people on like a "5 or below" on the depression scale.

"It doesn't work for everyone" does not automatically mean "it only works for mild cases". Those who see massive benefits from exercise (and those who don't) come from both severe and mild cases.

I'm not trying to nitpick here. It's just that depression makes people think their depression is somehow unique or significantly worse than everyone else's - and people commonly bush off the exercise thing entirely because "that's for mild cases of the weekend blues, not for real depressed people like me lying in bed pissing in Gatorade bottles".