If the instance owner doesn’t block it would we be able to block all threads (and twitter) on our profiles?
Yeah in mastodon you can block a whole instance. That’s why some servers are against threads but don’t want to defederate as they see it as the users choice
There’s also some sort of soft blocking option, which largely hides the instance AIUI but still gives the ability to follow users. This seems like a better option rather than completely defederating.
If Meta want to scrape your data they can do that anyway. All defederation does is stops you looking at their stuff (which, as you say, can be blocked at the user level anyway)