Oh.. so is what's happening is that the extension is loading all nostr short posts that have been posted by the user on primal (as when they do post on there, those posts would be tagged that they've been posted on primal / primal.net)?

Same logic applies to another nostr social site like Snort.social, i'm assuming, where the posts there are different than Primal's because the users' posts there are tagged with snort, not primal.