I write software that runs on tanks, its not bullet proof. Most the communication protocols just use security though obscurity. If you tell a gear box to shift form 1st to snip it'll do it, and break everything.
But when you considered air gap, and physical security surrounding it (12 inches of plate steal, 5 man team with guns, massive main gun), its pretty secure.
I'm thinking the joke is that, since the software runs inside the tank, which itself is bulletproof, the software is literally bulletproof since you can't shoot it with a bullet.