Seems like it is not a Microsoft deal and more like a Publisher deal... TR and Thief is 50% off on PSN too.
https://store.sonyentertainmentnetwork.com/#!/en-us/games/tomb-raider-definitive-edition/cid=UP0082-CUSA00107_00-000000TOMBRAIDER
https://store.sonyentertainmentnetwork.com/#!/en-us/games/thief/cid=UP0082-CUSA00252_00-B000000000000261
The sales could be really bad in both plataforms.