ActivityPub does not handle old posts. This post number is the number of posts since at least one person on that instance followed them. Before that, the instance didn't know they existed
That is incorrect. Mastodon is the one doesn't handle old posts[1], because they insist on presenting to the user only what has already been federated with it. See [2] for a more in-depth explanation.
If they would use the protocol as intended, and actually perform requests on the remote Outbox collection when the user wants to view the posts of a remote actor, there would be no issue.
> does not handle old posts. This post number is the number of posts since at least one person on that instance followed them.
That's not an explanation for the behavior here.
The instance's very first Hello World post <https://hexa.ninja/o/4473c26694414f928466337c1a9e0fc6> appears, while more recent ones don't. There's probably no one actually following this account from this instance. (Hard to check.)
ActivityPub is a publisher/subscriber protocol