In the first official trailer for The Avengers: Infinity War that debuted at D23, Thor actually meets the Guardians for the very first time — we know that because he specifically asks who they are.
If the MCU continues to move forward chronologically, then how could he meet the Guardians before Infinity War?
It’s entirely possible (and made even more believable by Brad Winderbaum’s remarks), but we will have to wait and see.
As of right now, one of the only real explanations is that the story of Thor: Ragnarok could somehow overlap with the story of the next Avengers ensemble.