That's what I had originally thought, but these were the responses I was getting:

"The heat tape or mat will heat the air as well, when you get the tubs only put a couple holes in them to start, that will help keep humidity and heat in, then make more holes if necessary. We got a couple of[...]
and they are great, they are so sealed that keeping heat and humidity in there really easy, and with the four latches they are very secure."

"Heat stays in tubs really easily, in my experience. Bigger, taller tubs will need additional helps, but ball python tubs seem to heat up fine with just a UTH+thermostat."

"[...]If your ambient room temp is around 70F and your humidity closer to 25-30 you should have no problems."

"my BPs all have ambient temperatures 7-10 degrees warmer than my room temperature in their pvc cages and sterilite tub, each heated only by a single UTH set to 90F. while a UTH is not designed to provide ambient heat, it will provide some, and the plastic enclosure will retain it."

Which is why I ended up going with the tubs. Now I'm due to get the snakes in a couple days and not sure what to do. I have a spare 10 gallon tank and heat lamp I could use for the juvenile, but then the humidity is going to go to crap, and I have nothing else to put the adult in larger then a 20 gal. Either way it's still better then their current situation, but still