It's on the author to advertise how they'd like communicated with.

A phone number != an email address != a WhatsApp/Signal/Telegram account != a Twitter/Facebook/Mastodon/etc. handle != a specific handle on a specific Discord/Slack/IRC service != "I go to the Lamb and Flag in Punterby on Wednesdays, maybe see me there"

It's irrelevant whether a low level protocol or its clients deliver everything or not. What matters is human A reaching human B. If human B wants to be reached, they will tell people in advance what their communication preferences are. And they will recognise that not all communication methods are fungible nor want to be or can be made to be. You can't expect ActivityPub to have the same behaviour as SMTP any more than you could IRC.