I agree. I'm not aware of a single sub that has wholesale moved to a lemmy instance. There's def a big increase in fediverse usage, but to say entire communities have migrated already is beyond hyperbole, it's just marketing speak.
there were still people on Digg for over a decade after the "digg migration". I think you're taking the term a bit too literally.
besides, migrations in software work the same way. You don't cold turkey drop the old tools. You make bridges, start weaning people on new tools, and phase out the old. That's the concept behind deprecation models.