The peak of the Mastodon migration was mid November, reportedly around 400k per week. This sounds like a lot, however a) many have moved back to Twitter due to it not being what they expected and b) Twitter has over 300m monthly active users.
People here keep omitting that part.
Regardless, < 1% is not a lot of Twitter users by pretty much any stretch. I think people in tech or fringe communities are seeing a high uptake in their own groups and assuming that to be reflective of Twitter users at large.