The AGA commentator (Cho Hyeyeon 9p, at https://www.youtube.com/c/usgoweb/) had already been calling the game completely hopeless for Lee Sedol for nearly 2 hours at that point, estimating Lee down by at least 20–30 points before komi, unless by some miracle he could win the ko fight.
Right, all about one or two wrong moves by AlphaGo. He still had a chance around the time Lee started to play the bottom of the board. But AlphaGo did not fall for the trick. I believed the English commentator said Lee could build a ladder but wasn't sure if he meant to say whether that technique would defeat AlphaGo or not.
I did not watch that stream, but IIRC the first move Sedol played in the bottom was a ladder breaker, specifically because white had a difficult-to-see ladder that worked a move earlier than black's ladder on the right side. At that point, there was no way Sedol could win, so the commentator you referenced probably did not mean any such ladder would defeat AlphaGo.
Edit: M5 was definitely played as a ladder breaker, so the above is correct.
As a 3d amateur, I'm really curious about when he resigned. It really seemed like he was playing the position out to go for the win (or perhaps to see how AlphaGo would fare in ko). It didn't look like he was searching for a place to resign.