You seem to be incorrectly stating what is on Wikipedia, which leads:

The fediverse (commonly shortened to fedi)[1][2][3] is a collection of social networking services that can communicate with each other (formally known as federation) using a common protocol.

That last bit is absolutely key: a collection of services using a common protocol. Imagine if two different email servers didn’t both speak SMTP. Imagine if two different web services didn’t both speak HTTP. The Internet as a singular entity is only made possible because of protocol interop between all of its constituent parts.

To say “the fediverse” is comprised of multiple incompatible protocols goes against that grain entirely, and to go back to pre-ActivityPub-as-W3C-specification days as an argument that it’s fine to label multiple incompatible protocols as all being components of “the fediverse” is really a stretch.

To me, this isn’t a let’s-agree-to-disagree-issue, honestly. While the term “fediverse” is arguably colloquial and doesn’t necessarily imply any specific technical attributes, it ceases to be useful as a term if Fediverse Platform A cannot in any way communicate with Fediverse Platform B because the two platforms happen to be using 100% incompatible protocols. Aside from a third-party bridge, the AT protocol used by Bluesy is 100% incompatible with ActivityPub used by Mastodon, Threads, and others. Therefore, they cannot both be simultaneously services in the fediverse.