Single Comment

Actually, it's a pretty simple explanation. Just as in FIDE chess there is a condition called "opposition" in K+P vs K endings which prevents the pawn from successfully promoting, the same sort of thing happens when you try to get your king onto the same 2D level in Hype. The opponent's king stays as close to your king as possible, in both a neighboring big and little square, preventing your king from ever actually getting on the same board as your opponent's king to hold it to a specific 2D level. The position shown is the minimum force needed to force your king's way onto the same 2D level as the opponent's king to get the hold. Once held, yes, even a rook and king deliver mate. Hope this answers your question adequately.

Ben Reiniger helped me find this position by playing the lone king as I looked over various possibilities. So let me here publicly thank him for his patience, as he didn't have my certainty there actually was a solution.

And finally, there is no requirement for the king which initiates the hold to stay on the same 2D level. This only applies to the king being held. So the lone king, being the one doing the holding, cannot be forced anywhere in particular other than to any safe square.