Very interesting game, i have never noticed this before.
It appears to me the pawn is checking the King, and also attacking the pawn on a2.
It would be good to know what the author says about the Knight. Hans says 'The rules do not state exactly the way knights move. One could assume a knight moves one square orthogonally, and then one square diagonally, skipping again attacked squares.'
This is one way to describe the Knight move, but some people describe it as moving 2 squares orthogonally then 1 to the side, or even moving 1 diagonally and 1 orthogonally outward. If you assume it moves like Hans says, then it does appear it is checking the King.