sitting on that bench waiting the bus is the first time aziraphale really has to come to terms with that he is now technically fallen. he hasn’t fallen in the literal sense but its just occurred to him he can’t ever go back to heaven. its not even a case that he’s want to anyway but it was always an option before.
for centuries he’s lived in this little bubble surrounding heaven and the angels because he’s one of them, absolutely, but he doesn’t want to go back and serve with them at all. he just wants to go for lunch with his friend and look after his bookshop.
nothing is going to change in what he wants but it is suddenly more real when crowley says “we’re on our own side” now. unlike the bandstand where he still had people to answer to and thousands of years of servitude restraining him.
as aziraphale sits and waits he realises he really isn’t part of heaven anymore (but nor has he become a demon). him and crowley are of their own stock in a new and terrifying way. they’ve both fallen from their sides, neither angel or demon but their own almost human side.
aziraphale has absolutely fallen but not in the burning, terrifying way he thought he would. he came down slowly and landed before he even knew what he was doing (whilst crowley flew upwards) and it isn’t as terrible as he thought it would be.