Pawn and Two: <Richard Taylor> You are correct. The players agreed to a draw, but as you have noted, Black is winning after 53...Rb8.
This turned out to be a very important draw for Flohr, as he would win the tournament by only 1/2 point over Alekhine!
In his notes to this game, Thomas was aware of his missed chance, for he stated, <Draw agreed, but Black can win by Rg8+>.
Thomas's suggestion 53...Rg8+ also wins, but your suggestion of 53..Rb8 is even better.
If 53...Rb8, then 54.Kf7 Rb7+ 55.Kg6 Rg7+ 56.Kh6 Rg1 57.h4 Kg8 58.h5 Kf8 59.Rxb4 c2 60.Rc4+ c1(Q) 61.Rxc1 Rxc1 and Black wins.
Other tries are also winning for Black. If 56.Kf6 Rg1 57.Rxb4 c2 58.Rh4+ Kg8 59.Rc4 c1(Q) 60.Rxc1 Rxc1, or if 56. Kf6 Rg1 57.Kf7 Rf1+ 58.Kg6 Kg8 59.h4 Rg1+ 60.Kh6 b3 61.Rc8+ Kf7 62.Rxc3 b2 63.Rb3 b1(Q), and Black is winning.
After Thomas's suggestion of 53...Rg8+, Black will win, but is it a tense and interesting ending. Then 54.Kf7 Rg7+ 55.Kf8 Kh7 56.Rxb4 Rc7 57.Rb1 c2 58.Rc1 Kg6 59.Ke8 Kf5 60.Kd8 Rc4 61.Ke7 Ke4 62.Kf6 Kd3 63.Kg5 Rc5+ 64.Kg6 Kd2 65.Rxc2+ Rxc2 (not 65...Kxc2 which only draws) 66.h4 Ke3 67.h5 Kf4, and Black will win.
Flohr also had winning chances in this game. At move 32, White could play (2.34) (18 ply) 32.gxh5 g5 (if 32...gxh5 33.Qf4!) 33.Ng4 Qe7 34.Rf3 Rf8 35.Rdf1, and after (3.30) (17 ply) 35...Ne8 36.R3f2 f6 37.exf6 Nexf6 38.Qxg5 Qg7 39.Qxg7+ Kxg7 40.Nxd5, White is clearly winning.