I think the point is that on a4 the pawn is easily targeted by a bishop on d7 (with the additional benefit of being able to play Ba6 as well to target c4, something that can't do if the pawns are on a5 and a6). A white pawn on a5 can't really be attacked by any black's minor pieces if by some miracle all the heavy pieces get traded.
Yes! I was hoping you would cover this amazing game.
I still don't quite understand Yip's a5. Why is the white pawn more of an endgame target on a4 than it would be on a5?
I think the point is that on a4 the pawn is easily targeted by a bishop on d7 (with the additional benefit of being able to play Ba6 as well to target c4, something that can't do if the pawns are on a5 and a6). A white pawn on a5 can't really be attacked by any black's minor pieces if by some miracle all the heavy pieces get traded.