The email is not the relevant part. Apple sign in requires the purchase of an apple device so it's almost guaranteed to not be a bot. However apple cant know (and shouldn't know btw) if the iphone user uses it to sign up and spam various websites.
Apple already has a lot of anti-spam/fraud accounts because they need to prevent iMessage spam (is your forum going to be subject to the same spam account pressure of a system like iMessage or iCloud?)
But also any argument that the makes spam harder to block also applies to Facebook or google logins - I can’t imagine spammers are using real email addresses with those services.
The only people this changes the experience for are actual users. Spammers already have a huge array of options to get “real email addresses”.
If you watch the presentation or read the transcript, they explicitly say that they're doing fraud/spam account prevention.
Do you really think you're doing more fraud/spam detection and prevention work than apple?