Papyrus holds his ground until there's no ground to be held, when the repeated arms pummeling the skybridge shatter it beneath them. He pulls his blue magic off of the Mother and back to himself, reflexively controlling his fall away with the plan of slowing close to the ground. It's not flying, but it's practiced enough to be comfortable.

Of course, that doesn't help with all the debris, or reassure Carly or anyone else watching that he's falling in a mostly-controlled way. Especially not when he sees her panicked expression, assumes she's struggling to fly, and reaches out an arm to grab at her as they approach the ground.