"Ideally, you should be able to follow accounts from the fediverse in Threads..." WTF. If you can't do this, it's a roach motel. Lock-in remains. Big accounts will know they can't ever leave and have a large audience. All positive feelings about them joining was built on this, that they were bringing a huge audience to us.
What's hard about it? I doubt it's technical. Meta just knows it means they'd be losing out on exposing people to algos and ads and data mining them.