It was definitely not obvious to me! What do you make of the third chord under ‘composing’ that would not be playable in a diatonic (?) scale that was used at the time of King David, as the video claims?
I've not seen the video, but that chord is just a usual major chord, that is, a chord with 3 notes: a fundamental, a major third and a fifth.
It's a very common chord, we could say it's a "natural" chord because when a string vibrates, its first harmonics make this chord. I'm not sure why it would be a secret chord, or why you couldn't get one from a diatonic scale. Now, it's true that, assuming your instrument is "stuck" in the C major scale, you won't have a G# note, which is needed for this E major chord. Whether or not they could play such a chord progression at the time of King David doesn't feel relevant to me, in the context of the rest of the song. I would be surprised if this is what Cohen was getting at by "secret chord". I don't think "secret chord" means a chord in particular in the song. If I had to guess, I would rather think it could be a metaphor of the search for perfection in composition, and this song would be a "tribute" to perfection, but not perfection itself. Just like Tenacious D "tribute" song :)
It was definitely not obvious to me! What do you make of the third chord under ‘composing’ that would not be playable in a diatonic (?) scale that was used at the time of King David, as the video claims?