It's similar to Valve and Dota2. We all know what it means but officially, it isn't actually DotA and doesn't stand for anything. They will never refer to the game as Defense of the Ancients. Seems to have worked for them.

Worth noting that the Atari of today is a shell corporation that has precisely zero to do with the original.

IceFrog was on the Dota2 team, and valve owns the trademark