! Isn't Necessarily Better

After multiple digressions related to Fischer - Gligoric, CT 1959, it's time to get back to the game itself. In the diagrammed position, White played 17.Nde2. Fischer gave his move a '!', and noted,

Black holds out after 17.g5 hxg5 18.hxg5 Nh5 19.f4 Qc5, threatening 20...e5.

Kasparov agreed with the '!' and repeated Fischer's variation.

1959 Candidates Tournament (round 4)
Gligoric, Svetozar

Fischer, Robert
(After 16...e7-e6)
[FEN "2r2rk1/pp1b1pb1/3ppnpp/q7/3NP1PP/1BN1QP2/PPP5/2KR3R w - - 0 17"]

Black played 17...Rc6, which Fischer passed over without comment, and the game continued 18.g5 hxg5 19.hxg5 Nh5 20.f4 Rfc8. Kasparov, however, assigned the move 17...Rc6 a '?' and noted analysis by Huebner:

17...Ne8!; Avoiding the opening of lines on the Kingside was essential: 18.f4 (18.g5 h5) 18...Qc5! 19.Qxc5 (After 19.Qd3 b5! Black's counterattack looks stronger.) 19...Rxc5 with approximate equality.

Indeed, after 17...Ne8 18.f4 Qc5 19.Qxc5 (19.Qd2 b5 doesn't look any better than 19.Qd3 b5) 19...Rxc5, White has nothing special and I can't imagine how Fischer could have maintained an edge. Since White has no particular advantage here, it looks like the move 17.Nde2 doesn't really merit a '!'.

Going back to Fischer's note on the diagram -- 17.g5 hxg5 18.hxg5 Nh5 19.f4 Qc5 -- White can play 20.f5, when 20...e5 gets Black into trouble after the nice 21.Nd5 exd4 22.Ne7+ Kh8 23.Qf3. Better would be 20...b5, when White has 21.fxg6 fxg6 22.Nce2 or 22.Ncxb5. This is no worse than 17.Nde2 Ne8, or am I missing something?