Pawn and Two: <keypusher> 25.Rxa8 is White's best move according to Fritz, and was also recommended in the 1912 "The Year Book of Chess" and later in "Lesser-Known Chess Masterpieces: 1906-1915", <25.Rxa8 would have been better, but even then Black would have retained his advantage.>
Fritz indicates: 25.Rxa8 Rxa8 26.Nxc7 (-.54) (18 ply) 26...Ra3 27.Rf3 Bd6 28.Rc3 Ra1+ 29.Kf2 Bxh2.
Fritz indicates after 25.Rxa8 Rxa8 26.Nxc7, an almost equal position (-.09) (18 ply), for 26...Rc8.
Your question why Johner resigned is difficult to answer. Certainly in the final position White can put up considerable resistance.
One thing to consider is in "Lesser-Known Chess Masterpieces:1906-1915", with the game score taken from, "The Year Book of Chess", White's 25th move is given as 25.Rc2.
I believe 25.Rc2 was the move actually played in the game. This move is more logical than 25.Re2, because if White had played 25.Re2, Black could have immediately won White's Knight with 25...c6!.
However, even with the move 25.Rc2, it appears to be a difficult task for Black to win the final position.
Black does have a considerable advantage, and I am fairly certain someone can work out a winning line. Here is one possible continuation that seems to offer Black winning chances: 28.Kf2 Kf8 29.Rd3 Ra5 30.Nc3 Bxc3 31.Rcxc3 Ra2+ 32.Kf3 exd5 33.exd5 Rd6