Player A remains on planet A. Player B remains on planet B. Player C commutes between planets A and B, accelerating hard enough for measurable time dilation.

Yeah but by the time player C reaches the other player A thousands of real years have to pass so neither player A or player B can meaningfully interact with player C or each other .

Time dilation is just shortcut to say you are no longer sharing the same frame of reference. If everyone has their own frame of reference there is no difference between single player game and multi player one