As the author of the substack himself indicates in the comments, this is not remarkable. The page is the official government POTUS account and followers are transferred, along with the account itself, to the new administration.
>"Ok so apparently follower counts on Facebook do transfer over followers for the official POTUS AND VPOTUS pages on Facebook. Twitter has different rules."
Users looked into this and it's not what is happening: "That would be a reasonable explanation for the auto-follows. Unfortunately that's not the case. People are being automatically added as followers on brand new pages of the president, the vice president and the first lady. If you try to unfollow, Facebook makes you follow them again. The only way around it is to block the pages."
Interestingly, Meta’s other property, Instagram, used the opposite approach. The old @potus account was renamed to @potus46archive with @potus being treated as effectively a fresh start.
The IG approach seems like the right one IMO — odd there wasn’t consistency there.
>"Ok so apparently follower counts on Facebook do transfer over followers for the official POTUS AND VPOTUS pages on Facebook. Twitter has different rules."
https://www.facebook.com/POTUS/