Yes very significant.
Go is really really hard for a computer.
Is winning Chess, Jeopardy! and Go significant? YES
Is any of it a milestone for real AI? NO.
We don't even know what AI might be. This is specialist programming. If we had real AI, the SAME program could start learning and eventually be the best at every game, as well as many other problems.
A rook (crow) or a baby that can't play chess, go or answer Jeopardy! at all is smarter. This is searching a database for solutions using a program written by very smart humans.
ALL so called AI is marketing of specialist interfaces to databases, often for a single domain per AI program (Driving, chess, go, bridge etc)