"So is every other piece-making tournament. ..."

No. The one that just finished, 0.54 themed, had the rules for creation very clear, everyone knew what he was supposed to create. Judging / scoring is opinion of each judge, that is not the point here.

"there is no game mechanic that is even close to as vague as your 'backstab'."

Unblockable for example, very commonly used (yellow squares, e.g. Wizard+). Does it mean pathing is unblockable ? By other pieces in straight line, or unblockable by not being / requiring straight line ? Or attack is unblockable ? By some property on target, that maybe blocks non-unblockable attacks (you don't know of existence of armor from description of Unblockable either) ? Or by things other than on target, that in other circumstances can block it (you don't know of existence of shieldsman / guardian from description of Unblockable either) ? Is it not blockable by some status on attacker (petrify, freeze, similar to Unstoppable on Vampire, you don't know anything about either of these from description of Unblockable) ? Unblockable by other own property of attacker (e.g. overriding augment, that Chastity has, just for yellow attack squares, but not regular ones, you don't know anything about either of those from description of Unblockable) ? Unblockable by immunities / enchant (on target ? on you vs king ? ) ?

So, paraphrasing you, there was nothing in my suggestion that was even close to as unclear or wrong as you are.