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 because Mastodon has a different structure. While Lemmy tries to mimic Reddits structure with individual communities, Mastodon follows Twitters structure as a larger hub more.
Mastodon, being less “subdivided” therefore can afford federation with such an entity with less risk. Their collective mass being able to offset threads weight more, in comparison to Lemmy where each community has to compete individually with Threads.