Edit: I'll get to it (my instance is small, I'm not very knowledgeable about the nitty gritty stuff, can I do it from Jerboa for example, or is it in some config file? I remember putting someone on the whitelist and thus blocking the whole fediverse...)
Because the people signed the pact did it long time ago, before any details about Threads federation was known. It was a typical fedi kneejerk reaction.
The key detail about Threads is that it's owned by Meta. That's the reason to block Threads. It was known back then, so there's nothing silly about it.