I joined Mastodon rather than I
    joined the ActivityPub protocol.

To me it is the other way round. You joined an ActivityPub node that runs the Mastodon software. If you are @denu@x.com and I am @jona@y.com - how did we not join the same network, independent of which software x and y use?

If x or y changes their software from softwareA to softwareB - did one of us then "leave" the "softwareA network" or the "softwareB network"? I would say no. We can still interact just like we did before. We are still on the same network, as long as x and y speak the same protocol.