Gypsy: <zydeco: 19....Qf5 looks better than Rc8 and 21....Qa7 looks much better than Ne6 (maybe Flohr preferred to lose the a-pawn than deal with white's activity after 22.e6. ...>
I think you are overlooking <20.Qd3! f6 21.Qa3 Qa7? 22.Nxh7!...> where Black ends a pawn and exchange behind.
To really appreciate the subtle point behind Boleslavsky's <20.Qd3!...>, let's consider first the immediate <20.Qa3 h6!>.
Black gets off the hook fairly easy on the account of <21.Nh7? Re8 ...>. And, of course, after <21.Nf3...> Black has the time to protect his hanging pawn at a6.
In contrast, let's consider this line <20.Qd3! f6 21.Qa3 h6? 22.Nh7!...>.
Here, <22...Re8? 23.Nf6+...> is not playable at all as Black drops the whole exchange, for instance, to <22...Kxh7 23.Qxf8...>.
All in all, surrendering only his <a6> pawn is the least material concession available to Black.