I could imagine it’s a very oldschool approach.
as rasturai already said:
“[…] with all the pad work and sparring that is a lot more important then the bag a couple weeks out”
one could imagine that a unit of heavy bag could, in theory, bring the mindset of some peaking fighters down a bit, who’s reflexes and movements are getting fine tuned.
Wheras you know, a bag workout does slow one down mentally sometimes a bit in a certain way:
I actually remember one time doing the heavy bag and then immediately sparring with someone who unexpectedly invited me. It was mentally harder to get in after I beat up someone who’s essentially helpless.
The first minutes I was definitely a bit slower because working alone always gives you the luxury of creating your own rythm and space whereas in fighting you’re sometimes competing for an inch.
Just my weak observation. But it’s really rigorously, anally oldschool.