First you are wrong. Riding at 40rpms up a hill is mashing. That is just how mashing is defined in common usage.

Secondly, you simply can't put a smooth constant force on the pedals when doing that. Low rpm means more force on the pedals do to the same work. Your body is built to apply alot of force in some and not so much in others. That means that applying smooth force or even force to both pedals in these maximal force situations is impossible.

Third, Even if you could apply a constant even force to the pedals which you can't it would stress the frame just as much. Pulling up hard on one pedal and pushing down hard on the other bends the BB out of line just as much as pushing down hard on one pedal.

Finally, sprint finishes in a track race are probably the highest rpm spinning cyclists commonly do. They aren't mashing because they are in a low force high rpm situation as opposed to your high force low rpm one. Want to see mashing look at a standing start or the jump in a mash sprint.