I really, really, really, really, really, really want to believe that the UFC is doing a 165lb tournament, but if that post listed GSP as one of the guys in it, it loses a lot of credibility for me. GSP wants to hand pick his opponents based on how big the name is, the likelihood that he would win, and whether it is for a belt. He's not going to get tied up in a tournament against unknown opponents. Probably the same is true of Conor and maybe a couple other guys that should probably be included in that tournament.
If I had to guess, I'd say Ali's tweet is Khabib vs. GSP for the 155lb belt with an outside chance it is for the inaugural 165lb belt.