Since as far as I can tell, Github isn't actually banning specific hashes (because they care about the content, not the hashes), and there's no sign that they're going to do so in the future, I can't say I understand the humor...
That was my initial reaction too, but thinking about it, it's potentially (probably not, but potentially) a violation of the ToS even if it's not automatically banned.
So, the hacker humor point is that you have a "poison" repo which, if you push it to GitHub, might get your account deleted in the future. You have no guarantee that it won't, for sure.
The way I read it, it seems to claim that github will just "insert" a known object with that hash into the new repository when it's pushed, though I'm somewhat confused as to how this would work, or how the submodule trick even adds the hash to the commit log in the first place.
EDIT: I read the article again and couldn't find any of that, so I must have read that in a comment here on HN instead.
Since as far as I can tell, Github isn't actually banning specific hashes (because they care about the content, not the hashes), and there's no sign that they're going to do so in the future, I can't say I understand the humor...