The most likely reason why SF9 or SF10 wouldn't do much better is that the Elo model isn't applicable: imagine if it's not about two engines doing similar thing

The most likely reason why SF9 or SF10 wouldn't do much better is that the Elo model isn't applicable: imagine if it's not about two engines doing similar things at different levels of competence, it could also be that AlphaZero's evaluation function is sufficiently different that it can exploit mistakes in SF's evaluation function. Then Elo(SF10) = Elo(SF8) + 100 wouldn't matter so much, it would give you SF's "strength", but not probability of winning against AlphaZero.