If I feed a 7hz input to some cheap hand-made thing like https://www.youtube.com/watch?v=liSEwqdq7aA , will it not vibrate at 7hz and thus produce a 7hz "tone" (disregarding that humans won't perceive that as sound, at least not the fundamental)?

No, because reproducing the fundamental is the thing. Saying otherwise is kinda like me saying I'm gonna take a voice call, run it through a filter that generates a ton of distortion harmonics, then seperate out those distortion harmonics, and then call it a "tone" of your voice.

But also the original post was about a 7hz tone somehow resonating with a chicken's skull cavity, which if you know the basic wave equation relating wavelength with frequency is an absurdity. The waves involved are multiple orders of magnitude too big to couple to a volume that small. They'll just diffract around like nothing.