I think most of them work very hard, year round, certainly as compared with me (and I go to the gym nearly every day, but not remotely like these folks of course). I've seen Nick Chubb videos, he's probably an outlier of course. I think the "laziest" scholarship player on Vandy works much harder than most of us in terms of phtsical activity/sweat/metabolism increase/etc.
No doubt many don't work as hard "playing school". Those muscles don't just appear from nothing.