The linked page specifically tracks Lemmy, although it’s not clear to me whether it’s tracking posts by users from Lemmy instances or posts to Lemmy instances, which is a medium-sized distinction (the latter would include kbin, Mastodon and other Fediverse users who are posting to Lemmy from their home instance, while the former would obviously include only Lemmy users).
There are literally, not exaggerating, over one billion Instagram accounts in existence. It’s self-evidently not the case that they have just silently registered everybody a Threads account and are counting those numbers.