I competed in a different sport internationally and lived at the Olympic Training Center for a number of years. Not hockey but I can weigh in a little bit.
Physiology is real and it is very much a thing. Sports science should certainly be adheread to.
But if we are talking about a team that has had multiple players say that they went to much better teams and practices are way harder, I don't think fans need to be careful about saying "practice harder" and don't need to worry about burn out.
Also these guys are top shelf athletes, they can handle a hard practice two days before a game. Sure you don't want to blow their legs off the morning of. But lets say they are on a schedule where they play Monday, have Tuesday and Wednesday off, then play Thursday, these guys can have a really hard practice Wednesday and recover.
For reference, a mid distance runner who spends their entire life preparing to try and win the Olympics. Working towards one day down the track, will likely go for a 60-90 minute run at least once per day, often twice. Some days will be significant work days. All while they are actively competing at the Olympics. Also, it is fairly common to train through events leading up to the Olympics while expecting to compete at a high level.
NHL players can handle hard practices.