You’d see comments and posts from their users on other instances that don’t block Meta.
It’s unclear how many users you would actually exclude, I think a lot of users who are on the fediverse right now don’t want to have anything to do with Meta.
As the fediverse grows, there will be different bubbles with not much interaction between those, mainly because some instances won’t be moderated while others will try to create discrimination free environments.
So it’s a browser?