It’s a real thing. I experience it every winter when I leave the basement of this polar country and go ride somewhere tropical for 2 weeks, then return to the basement. You lose most of it within 2 weeks or so though.
The bigger problem with it is how to you balance the stress of it on your body? Yes heat stress is extra stress. And it impairs your ability to hit your intervals.
And the question I don’t know the answer to is whether you would get faster by heat training but riding less intense, or not heat training but being consistent in your intensity rides.
I have lost fitness in the past during heat waves because of this tradeoff. I find it very hard to balance, and I think I would only deliberately try heat training again during a taper.
Does it work as well as just riding your bike more/better? Dont know.