IMHO, the point of both Delta and the new ActivityPub based chat (does it have a name yet?) is prevalence, at least in a certain part of society.

I remember, that Delta proponents said, that "you can reach everyone, who has an email address."

Now it is: "You can reach everyone, who has a fediverse account."

In the end, it is not that easy, e.g. because email users might not have autocrypt in their client, and it will take a while until there is #MLS in e.g. #mastodonEl.