You also have to keep in mind at that point Magus hasn't been weakened by Lavos yet so he has FULL POWER.
Plus, the bats being there it helps assumje magus.
I took a VERY VERY VEEEEEEEEEEERY close look at it, and it's obvious with the way it's made, it's supposed to be up to you who won, but if you REALLY can't decide for yourself, the thing has spiky hair at it's ends. So either it's Magus or Glenn changed back very, very quickly and it's going by the graphics he had in the animated ending.