You're conveniently forgetting that this "AI" is a representation of tens of millions of amateur plays which is far more than a few decades in total time. Not that Lee Sedol needs someone like me to defend him, but remarks like yours are very misleading.

A human is far, far, leagues, more efficient at learning than today's AIs. These AI requires millions of hours of man time of data to even come close to competing at the level of an expert person which did the same, and even arguably far better, in a "few decades".