Unable to understand why N.McDonald - A. Stromer (1991) is a draw and not a win [Title "Black to move"]
[FEN "3k4/3r4/8/8/p7/8/4K3/1N6 w - - 0 1"]

1... 58. Kc7 59. Nc3


McDonald notes that if 59...a3, then "60 Nb5 wins it." I don't understand why this wins it. Can't the King just deflect the check and continue the game? Or by saying "... wins it", does McDonald mean that after that, the game is practically a win as he can mate easily? 

If anything, I'd assume it'd just be another draw.  Can someone please clarify?
I suspect he means wins the pawn, as opposed to the game. A lone knight is insufficient material to mate, and black would have to cooperate to be mated with the rook vs. knight.