I might be misunderstanding what you're saying here. How is ActivityPub more authenticated than Webmention? WM requires the poster to host their content on a website. This is exactly what the AP spec says to do. Now, since the spec was published, most AP implementations also support HTTP signatures[0], but this doesn't provide additional guarantees that you can't get with WM. The authentication is still tied to a URL.
As far as spamming goes, I don't see how WM is any worse than AP. In both protocols your only options are passlists and/or blocklists.
[0]: And an old version that doesn't have an official spec. ActivityPub's issues with spec stagnation and de facto standards is a whole other thing.
As far as spamming goes, I don't see how WM is any worse than AP. In both protocols your only options are passlists and/or blocklists.
[0]: And an old version that doesn't have an official spec. ActivityPub's issues with spec stagnation and de facto standards is a whole other thing.