It seems people can slip off perfectly (Look at Yoshi BtP. Even though I think this stage maybe can be improved). They usually mess up with other things.
The final jump and up+B is the hard part on DK's. There's no margin for error. 1 frame out and you'll get malva00's WR.. Both the jump and the Up+B need to be on 1 precise frame.
Also, getting the maximum distance on the up+b before last isn't so easy. DK tries to fall off the stage after landing.
I think if it were easy malva00 or one of the other good BtP players would've got the TAS time earlier. Sometimes people think that a stage can only be done so fast, and then once they see a faster TAS run they end up beating the old record.
But really I don't know how hard it is. >_>
EDIT:
Luigi BtP 20"11
http://uk.youtube.com/watch?v=19Mien8buCc
Jumping and Right-Up+B is faster.