Yes, that's exactly right. "Covering but not fretting" a string on the guitar will dampen the lowest harmonic because the full length of the string is unavailable for vibration [1]; frequency is one over wavelength (e.g. twice the frequency is one half the wavelength). You can pluck either side of the string when covering it to produce the same harmonic (neck side or body side).

On a trumpet, the embouchure will affect the frequency of the vibration of the air compressed in the tube, and simply drop out lower harmonics, as can be confirmed via spectrogram.

The same thing happens on a guitar, leetcrew I encourage you to try the spectrogram linked at that site with your guitar to note the effect [0].

[0] https://musiclab.chromeexperiments.com/Spectrogram [1] I am under the impression that the fundamental tone of the string requires the whole string-length to resonate, and if it is clamped, pinched, or otherwise muted all you will hear are resonant harmonics that can exist on smaller string segment lengths.