Really any 'intelligence' exhibited by a computer is deemed AI, regardless of brute force or use of smart heuristics. For example, a chat bot can be coded to respond to most responseresponses using many, many if statements. This is an AI no matter how poorly coded/designed it is.
The chess playing computer beating a human professional can be seen as a meaningful milestone. I mean, someone programmed a computer to beat grandmaster chess players and chess geniuses. Many thought that wasn't possible since chess is such a complex game. This kind of work likely segwayedsegued into more complex AI, for if a computer could play chess, then it surely complete other complex tasks as well.
Note how refined chess programming is: magic bitboards, zobristZobrist hashing, pruning, lazy smpSMP, and many more. This is perhaps not the sort of milestone of AI that you thought, but again, the things that can be considered AI are pretty broad.