I think it's more about _letting_ them pick a different email. While I can understand that AnyList (or any other app for that matter) would want to, on occasion, send marketing emails to users, I don't think any app would, in their right mind, _require_ the user to provide a 2nd email address.
But by allowing them to optionally give that 2nd address, they can provide a path forwards with people being able to use Sign In With Apple (of course, that means some users may opt out of marketing emails entirely by refusing to provide a 2nd address).
This does probably go against the spirit of the feature, but if it actually is against Apple's rules to be doing this (anyone know the answer to this?), then it would definitely veer on the side of user hostility on Apple's part, since I would expect many apps to be taking a stance similar to the one taken by AnyList here.