I can see the possible information he garbled. I can easily see flies not generally flying over a few meters in height. Their food is generally low down, as is cover to hide in. If they flew higher then they would be at risk of both predictors (bats and birds) and cold, for no real gain.

There might have been a scientific paper that noted the fly’s (self imposed) height limit. “Generally like to stay below 2m” became "can’t fly above 6’ via junk science reporting.

I might be completely wrong. But I do find it interesting to try and reverse how the various insane “facts” that some people come out with come from.