Yes, Dota 2 is not a multiplayer poker game. I agree that the title is ambiguous, but it's not a stretch to imagine that "poker" is implied here.
I don't think it's implied considering the articles compares the poker bot to go and chess bots (which are the non-multiplayer games the title is referring to).
My guess is that by "multiplayer", they meant "free for all", as opposed to "N vs. N". In other words, multiple opposing factions.
From an AI and game theory standpoint, there isn't much difference between two-team zero-sum and two-player zero-sum if the teammates are trained together. That said, the Dota 2 work is extremely impressive for a variety of other reasons.
There is far more ambiguity when you are competing against five mostly-aligned strategies vs a single shared strategy.
Agreed, the "first multiplayer" claim needs some walking back or caveats.
Cool achievement, but hollow marketing doesn't make it better.