>about how polyrhythms are actually cords

Meant chords? I'm not buying it. The commonality is very stretched (notes are ratios, and chords are several ratios together). That's like saying "code is actually poetry, both are based on the arrangement of alphabet symbols groupped in small blocks". More something for a TedX talk, than something practicing composers and musicians have in mind when using either.

For starters, in chords the ratios are stacked vertically (multiple "ratio" notes played together at once), in polyrhythms horizontally. In chords it carries harmonic information, where polyrhythms can and are just as well be played with unpitched drums and percussion. And in general they serve different purposes. You can have chords playing without any polyrhythm in the rhythm side or play monophonic lines with polyrhythms.

I just watched the first half of the talk, and it's pretty interesting. I don't think Adam Neely is actually saying they're the same; he's saying it's interesting to think about their similarity. And what you actually get out of the talk is that his point is that pitch is our perception of frequency over a certain limit, and that rhythm is our perception of frequency under that limit. They're both perceptions of frequency and the ratios of those frequencies are the defining property of both chords and polyrhythms.

I haven’t seen the video (and likely will not) but there is a similar “shower thought” that vision is also energy of a given range of frequency, just like hearing.

I haven’t seen the video, but I assume what it’s getting at is that if you have like a simple 5 over 4 polyrhythm and you speed it up sufficiently you’ll start to perceive it as a major third.

I kind of also think that’s a little meaningless since while tempo for a beat and pitch for a tone are both sort of frequencies we perceive them completely differently.

Watched it. He does go into this perception difference, but he still makes a connection between the consonance of a chord due to the frequencies interacting and the consonance of a polyrhythm with the same ratios. A 3-4-5 polyrhythm is analogous to a major triad and has a pattern you can "feel" better than something dissonant.

You can also play chords polyrhythmically. Timpani is a great example.

You can, but it's optional (and orthogonal).