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 at least among people with shared interests. 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.
If the instance defederates, users do not have an option to interact with the defederated instance anymore. The only choice you have then is to join an instance where the moderation policy agrees with your values or host your own instance. If you just want to see if defederation was the right idea, you could just visit those instances directly and look at their local timeline.
You're not fooling anyone.