I just started a game with a single player in it just to get a feel for some things I felt I was often skipping when focused on winning. Immediately on start up the game declares me winner and gives me all the victory related achievements (no extra heroes, no magic, the speed run ones etc.) and I don't really think this is intended.
I get that it will always be possible to cheese achievements in games with as customizable setups as this and I don't think that is much of a problem in general, but the game should probably check if there were any defeated opponents before it declares victory.
Getting the any% achievement from a 0 sec game is pretty funny though.