I think that is a controversial statement. I agree that it is the received wisdom, but I am not convinced that it is accurate.
Bees ventilate the hive by fanning. Over millions of years, they have developed a method for lining up and creating a “laminar flow” of air to ventilate the hive very efficiently. Laminar flow only works if the hive has all solid surfaces. Hollowed out tree trunk hives do not have screened bottoms.
If you open up the bottom with a screen, the laminar flow becomes much less effective. @JeffH has a beautiful video of bees lining up to circulate air around his observation hive. I think a screened bottom would have made their efforts ineffective.
My summers are often in the 80s or low 90s Fahrenheit (28 to 35C), and with solid bottom boards or the screen in the upper slot, I have seen zero bearding, so I don’t think the bees are overheating. Of course my slatted rack may help (salted rat for @skeggley ) as it gives “hanging out” and fanning space at the bottom of the hive.