Your point would be true if ActivityPub was a properly defined specification, but it isn't. It is voluntarily vague in many places to accommodate for the differences in implementations, which means that each implementation needs to understand how other software works to interoperate. So in practice it is more accurate to say "Protocol like Mastodon", because even if you can talk with Mastodon it doesn't mean you can talk with Pleroma or Pixelfeed or others.
More details here: https://overengineer.dev/blog/2019/01/13/activitypub-final-t...
I must correct you here on "Protocols like Mastodon". The protocol that underlies the Fediverse is the W3C ActivityPub Recommendation [0] and making that distinction is really important. Not doing so would encourage centralization in the same ways as discussed elsewhere in this thread for Email vs. Gmail.
Mastodon is just a fediverse app that was an early adopter of ActivityPub, and managed to become quite popular. Part of this due to a better productization, and the jumpstart this gave them offering the best features first. To many newcomers Fediverse === Mastodon, but - while understandable - this is massively shortselling the interop capabilities of the protocol.
What has already happened by Mastodon dominance is that they are driving the technological direction, but not necessarily in the best possible way and to the overall benefit of the Fediverse. They have a custom client API for instance that is becoming a de-facto standard, adopted by others out of convenience.
[0] https://www.w3.org/TR/activitypub/