Somehow I was asleep at the switch, and I missed this play.
I'm not sure how the tie was broken, but splitting is much better than slotting here. Slotting with an opening of 51 is doubtful, but with the opponent having a second homeboard point it's probably a blunder to slot.
The problem with slotting with 51 is that there are no builders, so even when you're not hit you can't cover the blot often enough to make the risk worthwhile. Add in the additional danger of being on the roof against a two point board and slotting is severely counterindicated.