I am not sure if we have fediverse, or if we have isolated siloses.
I am asking if that is the case.
This is not 100% accurate. Who the collective members of a server follow affects the content shown in searches and feeds to some degree.
For example, compare https://hachyderm.io/tags/cybertruck to https://mastodon.social/tags/cybertruck
As you can see, the bigger server 50% more posts, 40% more participants, 25% more posts today.
As you surf your "local server", you don't get "everything" from the Fediverse. If you care about that, you likely want to join the "biggest" server you can.
Personally I don't care too much about this. There's more content added to the Fediverse than I have time to consume, and so seeing a slice based on who I currently follow and the server I utilize is plenty for me.
But it also makes sense if it bothers people who don't want to miss out on some zeitgeist.
A very small one has a large likelihood to disappear. Each instance has different moderation rules. On a small instance, you don't even see entire threads and lots of other stuff does not sync properly.
Even further: a user on a mastodon instance can see & interact with posts on Lemmy (the Reddit alternative), pics on Pixelfed (=Instagram), videos on Peertube (=Youtube), and content from a long list of other services, all from their single mastodon instance.
(There are exceptions, e.g. you could run a siloed internal company mastodon, and notably server administrators are able to block other servers entirely - but as a user you can always choose a server that federates as widely as you'd like, and if you self-host you're unlikely to block or be blocked by anybody)
_In general_ you may assume probably yes. Caveats are if one of the users instances blocks the other one's instances (in practice, rare) or if one or both instances doesn't federate at all (Truth Social and Gab are the big examples of this).
The user identity is an issue on the fediverse and it is know by the creators of ActivityPub. There are plans to create a way for global identities which will solve many of these issues.