I'm not sure I understand your question or why you think of those statements as being exclusive. I will try to answer separately.

is the fediverse a service that should allow Threads/Meta

The good thing about the Fediverse is that it isn't a single service, but many federated ones. For any single instance, I think not defederating maximizes user freedom: A user who wants to interact with Threads can do so while a user that doesn't want to see any content from Threads can block their domain.

is it like XMPP and doesn’t have to be popular

I think it's a little bit different from XMPP, in a sense that the Fediverse is a public space where I communicate with strangers, so I would like it to be popular. For instant messaging I just need my friends there, but sure it would be easier if I didn't have to show everyone how to create an XMPP address and what client to use.