Loading that in Mastodon's "Web" says it contains "4 posts", but consulting the "Posts and replies" tab will reveal the contents of 5 posts. Meanwhile, this number is about half of the expected number, based on the posts that are actually visible on the <https://hexa.ninja/> landing page (i.e. as viewed in e.g. Firefox).
ActivityPub is a publisher/subscriber protocol
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.
[1] https://github.com/mastodon/mastodon/issues/14017
[2] https://github.com/mastodon/mastodon/issues/14017#issuecomme...
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.)