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.
It's also interesting that nobody's asking "how is Threads federating with Mastodon/Lemmy?" All the focus is on the underdog but shouldn't we pull our heads out of the bubble and give some attention to what the big dog is doing?
They're probably using ActivityPub?