But PS4 in December was pretty supplied constraint if I remember correctly so it didn't have the Holiday advantage people assume it did.
Yes, but you also expect sales to always be high in Q4, so manufacturers and store buyers are going to load up with stock. Therefore, they had the supply to sell. (Shortages ended after a few months, so they weren't particularly severe.) It had a Q4 shipment advantage, then, as it was expected to sell a ton being the new thing (with XB1) and the holidays. Therefore, stores bought a ton to sell.
PS4 is still well ahead of NSW's pace, and it remains to be seen if NSW will keep up any momentum through the year. I don't think it really matters, though. Western third-parties have done a knockout job segregating the Nintendo game buying audience from exposure to their products, so a Switch success is unlikely to have much of an impact on long-term prospects for PS4/Pro and XB1/Scorpio, who rely almost exclusively on Western third-parties, at this point.