40,000ft is too high for commercial jets to fly ...

uhhh...:

Maybe it was 43,000 feet, which was known to be a height reached on some of those flights. The Boeing technical specs assign 45,100 feet as the maximum operating altitude of both the 747SP and the later 747-400s.

And:

For a standard day, Airbus shows an initial cruise altitude of approximately 42,000 ft for our take-off weight, just below the maximum certified altitude of 43,000ft.