'Speed damage' there refers to a worm hitting the ceiling (or some other surface) from below and coming back down. In this case Sensei's worm travels downwards and hits a wall, so it would be more accurate to call it regular fall damage.

I think speed may have some influence on fall damage which is where the confusion is coming from?

Anyway, assuming the nade did 46 damage I'm not too surprised by 14 fall damage there, you see that kind of damage in ZaR / roper a lot where the worm is on the side of the map, and hits the wall on the way down.