The conversation was "The ability to run on my phone seems like an important feature."
Either you have an iPhone or you dont. Sure they might have an ipad and imac and an android phone. I suppose thats possible. But at that point, you are the exact kind of customer this business model is designed to get to switch over to the full ecosystem.
Are we really arguing if the "Sign in with apple" button will work on websites from chrome on windows? If apple wants to be an identity provider, their web sso will work everywhere. Or are we talking about iMessage, the flagship iPhone app, not working on android phones? Apple will lose more customers to Android, who only want an iPhone for iMessage, than they will gain.