Allen extends the play ALL the time. The receivers know that they have to get into scramble drill rules ALL the time. Allen evades a rusher and extends the play very often. Yes, sometimes Allen rolls out as part of the design, but not all that often.
Mahomes used to be more off script, but their offense has changed. He has turned into a quick release, timing QB more often than not. He doesn't go deep very often any more. Part of it has been adjusting to poor oline play and less talented receivers for them. Yeah, he still extends plays and goes off script quite a lot, but I still think Allen does that more.