yeah that is nuts. Its not just sea of thieves, ALL new exclusive xbox one games - Crackdown 3, State of Decay 2 - will be available day 1 for "free" if you are an xbox game pass sub. Still, thats an extra $10/mo or $120/yr so unless they plan on releasing more than 2 games/year its still better to just sub to pass for a while, exhaust the library as you see fit, and then only resub if a game that you really want pops up.
since its digital it should also let you play on PC i think those new games, but not sure?
also god of war gets a release date, 4/20
God of War Out April 20 on PS4