Clearly I am singing.

Yes and my point is that the architecture that is most conducive to countering that tendency in physical acoustics suggests the opposite conclusion people come to about echo chambers in abstracted metaphorical application to communities and systems of people in conversation.

I genuinely think it muddles discussion around human topics when the phrase "echo chamber" is used as the association people make is so backwards it hurts more than it helps.

open spaces don’t echo because there’s nothing to bounce off of.

You are precisely wrong here, echoes require open space to proliferate.

Plus, if you shrank down to the size where you could fit inside one of the bubbles of acoustic foam, it may very well be echoey in there.

Isn't the reason you are invoking a contortion of scale to shift our focus to inside one of these smaller bubbles/cells motivated by a desire to induce a sense of some small degree of open space around us? In a sense, aren't you arguably still invoking the idea that space is what allows echoes rather than density and enclosure?

Singing them! It is only natural, they are full of notes.

But a canyon won’t echo regular speaking voices like a cathedral does

...because a Cathedral echoes more than a Canyon does! Which is my point! When people apply the metaphor of the echo chamber they almost always use it in precisely the incorrect mapping and speak of mitigating the "echo chamber effect" by creating large, enclosed spaces with huge unbroken internal voids where even whispers cascade all the way across the volume in a cacophony.

Conversely when people speak of systems of communication that are most like open celled or closed celled acoustic foam with many smaller spaces separated by manifold barriers and divisions, they are likely to refer to it as being prone to the "echo chamber effect" and I think it seriously hurts rational thinking on the subject.

I have never seen someone criticize a human communication system with the structure of a Cathedral as an "echo chamber" while gesturing to the structure of a Canyon as superior in that it lets echoes proliferate but doesn't concentrate them. Rather, it seems to be used almost without exception to argue in favor of making systems MORE like a Cathedral and less like acoustic deadening foam in structure.