What City require from their goalkeeper simply has less to do with shot-stopping than it does at other clubs, because their goalkeeper faces far fewer shots. Horses for courses.
I mean it's easy to say "if he wasn't at a side as dominant" but he is at that dominant side and his skillset is part of what makes them so. The fact that he'd let in more goals at a weaker side he was less suited to is hardly relevant, as you're supposed to have players who suit your team's specific needs. He'd do worse in lesser teams just as most of the goalkeepers who stand out at lesser teams would do worse at City. It is what it is.
Alisson is indeed obviously the better goalkeeper though.