I'll give this a try - the Trisolaran's method of communication is very direct, and they have nothing programmed in their brains to detect lies, subterfuge, or nuance. So while their sophons were able to intercept forms of communication they had no way of 'detecting thoughts', so to speak.
I can't quite remember from the books but I think the data retrieved from Mike Evans was how they first clued into the fact the trisolaran's couldn't read minds or pick up on deception, but also that Evans told them more about human nature and it spooked the trisolaran's to the point where they'd no longer communicate with evans. This discovery leads to the the Wallfacer project, and is where I found nothing was really explained about the wallfacers in the Netflix series.
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