It might be specific for each person but for me I believe it.
This summer I did hard workouts in mid day in 90+ degree and high dew point. Then my family and I went on a long vacation to a 7k+ altitude town in August. I normally feel like sh*t the first few days at altitude and expected to not be able to run at all but the first day there I felt fine and did a 13+ mile run without issues. Has to be that there is some adaptations from heat training that cross over to benefitting at altitude.