Yes but isn't that entirely the point?
If our opponent is playing so that he is exploitable we want to adjust our strategy to exploit him, if his counter adjustment puts him more towards a GTO then Sonias play also become more GTO.
Basically, against all but brain dead opponents Sonias ranges would be balanced, for a certain value of balanced.
I guess the counter argument is that if Sonias is adjusting her play, then given a random run of cards from a truly GTO player against Sonia, if Sonia adjusts at all, she should be a losing player (if only marginally).
I think there are 2 things to take from it, we can't look at a hand from Sonia in a vacuum and expect it to be the GTO way to play the hand.
But the adjustments from GTO play Sonia does make vs. a player like Polaris are probably very minor so if we play our ranges like Sonia we are most likely not making any measurable mistakes.
All I am spending a fair bit of time on saying is, if there is any merit to checking back a balanced range on certain board textures from a GTO standpoint, then I think in the 'real world' there is an even greater reason to do so, because not only can playing closer to optimal never hurt us, but we are also putting our opponent into a situation he is likely much less familiar with given that checking back flops HU is not the ordinary play.
I agree with the last section.
But earlier on you said that if someone would play balanced then Sonia would play close to balanced too (right?), but I don´t see that since if someone who plays balanced ch/r a flop just at the right threshold for us to be indifferent to call down or fold. We might just call down every single time since our opponent doesn´t adjust. So as long as we don´t make stupid mistakes we can play in any way we want against a balanced player. Because he has already neutralized the EV of all decisions (except the braindead ones)

A
3
(3.7 BB, 2 players)