I'd have to check what it runs, cause maybe.But Flamethrower is a 3HKO. Fire Blast is 2HKO but accuracy. If it uses Overheat, say on switch or w/e, chances to 2HKO go down. If Flamethrower then Overheat Regice dies:( Bit worse than I remembered at max SpD but I'm very biased. Ancientpower from 4/0 Regice vs non-bulky Zary Y(why would it be bulky?,) is a guaranteed 2HKO.FWIW, the most common Xurkitree set, z-hypnosis will just set up on Regice and win the game straight from there most of the time. Charizard Y will pretty much win at full health every time vs. Regice as ancient power does not KO due to Zard Y's suprisingly good special bulk. Not that I think Regice is bad by any means, but it has its flaws that keep it out of mainstream play.
Yes, Hippowdon in general is probably a windup for that team, though there is certainly things that Hippo does not appreciate such as Kartana, z-draco from naga and mega gyara.
Tl;dr they don't like each other they both hurt the other bad but no OHKOs anywhere.
EDIT: Dang it checked Pyukumuku's pokedex entry cause was thinking BP to Ingrain Celesteela....but it learns no attacks xp. Idk why they did that, probably too ridiculous for even me to use now, or is Taunt not that common anymore? Some interesting moves and Unaware is good, but it's stats aren't that good, Registeel is better everywhere for example. It shouldn't be gimped this badly on moves.