Well, I think it is mostly hokum. The brain and body of 18-25 year olds is close to at it's maximum in terms of energy reserve, recovery, and output.
Playing 40 minutes with mini breaks whenever a foul is called, larger breaks when time outs are called and between quarters, and a larger break for halftime, and moments of inaction each time a team walks the ball up court is not that grueling for great athletes. Add in a few minutes on the bench in most games. All the players I have heard talk about energy levels have said that they look forward to game days as the practices are much harder and longer than the games. And the coaches know this so when they get a stretch of 4 games in 8 days, practices get cut as does the prep time for games. That in itself makes the games more difficult to play because they haven't been able to put the work in to adjust their game for the next opponent. The advantage they have is they play the same teams twice each year in conference, and most of the OOC each year are the same teams, so their prep work for most is a reminder of previous prep when the season gets compressed into these stretches. (And fairly evenly sprinkled in with the 'hard games' are much easier games where the starters play 30 minutes or less.)
One of the reasons Uconn has such tough practices is specifically to stretch mental and physical endurance in the team and the success shows in games. Uconn players don't gasp for breath, signal to be given breathers on the bench, or double over clutching their shorts during breaks in action. Another reason is muscle memory and mental recognition of patterns - the repetition at game speed over and over is not by accident, nor is the yelling. I think it was Sue's famous quote, 'We don't practice until we get it right, we practice until don't get it wrong.'
This year we have injuries, Breanna famously put out a call for walk-ons one year because the roster was depleted, There was a year where Maya and company had only seven players available for the year I think, and those years were championship seasons and no one talked about endurance. Now the rosters those years were fabulous and this year significant talent is injured, but the starting five all look very good to me. We have a Lieberman nominee, a candidate for AA/NPOY, a candidate for comeback player, a leading percentage shooter, and a 5th year player flirting with averaging a double double. The fact we have a NPOY and a top ten freshman sitting out the year, and two other high quality players injured is tough luck but the team playing is seriously good.
Mental mistakes and physical fatigue are the things other teams suffer and the reason Uconn has more undefeated seasons than every other D1 team combined. Does Uconn play less than perfect on occasion, yes. Do they occasionally lose to better teams, of course. Do they occasionally lose to lesser teams, very rarely. Every year seems to have a doldrums stretch when the sun barely shines and the campus is on break but that is not physical or mental tiredness in the way people seem to talk - I think it is more simple lethargy and a human desire to hibernate when you wake up in the dark and eat dinner in the dark, and spring seems so very far away!