Says who?
You misunderstood the terms. Blizz counts 1st-4th as “winning” in terms of quests and achievements. It has nothing to do with MMR. You can “win” and still lose MMR, as you found out. That doesn’t mean you didn’t win. That simply means you didn’t beat anyone of significant MMR, and the people who won WITH you - albeit 1st through 3rd - were people you should have beaten.