Yes, there is a string vibration at the fundamental frequency, as Scott Murphy showed in a high speed video, looking at the string at the bridge pin. He also demonstrated that the bridge pin moves at that frequency. However, audio analysis shows that the fundamental frequency as heard (by a microphone, or by the ear) is very weak, really imperceptible among the various upper partials. Presumably the weakness is due to the soundboard/bridge system's inability to project that frequency.
So, yes, the pianosens sensor is verifying the presence of the vibrational frequency, but that doesn't mean the fundamental is present for practical purposes (ie, heard by the ear). Your "proof" applies to string movement, as that is what pianosens is measuring. String movement is not sound.
Sound at the fundamental frequency can be present in large pianos, but it is so weak that we can say that it is essentially missing relative to the rest of the spectrum.
Didn’t you see the mic results also? I think you did not follow me. I re-did the mic experiment that I did 20 years ago, same result. As a matter of fact, the mic experiment showed that the ratio of the fundamental to 2nd harmonic was even a little greater that it was with the sensor, furthering my point.
My major frustration with the PTG forums is 'lots of conjecture and opinions' but 'lack of experimental data'.