Based on data from https://fedipact.veganism.social/ it seems that the majority of instances blocks threads.net. I'm sure there's Lemmy instances with either approach that have slipped through the cracks as the list is a work in progress.
The percentage of users doesn't correlate to instances as the biggest instance on the Lemmyverse has roughly 3x the users of the second largest, a NSFW instance, 4x the users of the biggest "niché" instance and 5x the users of what I see as the second largest general purpose instance.
At the moment, Threads runs from one domain. Do you seriously think it's going to stay that way? They also have 100 million users, compared to less than 2 million across the entire fediverse. Once Threads gets serious about federation - maybe a year or two from now - what you're suggesting is going to be like constant whack-a-mole. That's in no way a fun prospect.
And even if they stick with one domain (which they won't) - the hate groups on Threads will still be producing content that other fediverse users will be talking about/linking to/quoting etc. The fediverse is very quickly going to become a miserable place to be for those who thought they had found a place where they could be free of all that harassment.