Don't think it's AB. I once heard his training schedule and it wasn't brutal...far from it. Tbf I can understand that t.i.c his style. Solid strength training tho.
Just look at the most conditioned. It’s guys like Crawford, Beterbiev, Oshaquie Foster. These guys come in carved out of stone and they don’t neglect their boxing training neither. Mayweather did mainly light work, for very long, it’s no wonder his body broke down and he required copious amounts of drugs to maintain. Workouts should be brief or you waste away too many chemicals and muscle. No one needs to be training 8 hours a day.
Had a chat about this with a friend years ago, I think the hardest trainers in boxing are guys we've never heard of, ones who probably juggle it with a full time job, hungry and desperate for their big break