The magic moment is importing your secret key into an alternate client and all your contacts, posts, and feed populate from the data stored in the relays.
Which is to say, all of the potentially intrusive human information gathering and spying that you can encounter on twitter, but now cryptographically signable and EASIER to move from place to place.
Perhaps the ability of your mastodons and emails to lose your info might be just as much a feature as a bug.
So you can do it, but it’s not really designed for that use case.
I would be great if there were a full-featured single user ActivityPub server, but last time I looked, there wasn’t really.