I wager your talking more about BPM than the pitch of each note. Perhaps there's some argument that lower tuning naturally gives rise to lower BPM when composing a song, but I'm skeptical of that. If you take a song and transpose it or detune it a bit, you don't mess with the rhythem do you?

So if we ignore the tempo; What makes the flattening of a note "calming"?

If anything I might become a bit excited or agitated by the breaking of expectations. Though 8Hz is somewhat subtle.

I'd be curious what songs they used for testing, since I'm thinking that some songs are simply calmer than others at 432Hz for whatever reason. Or visa-versa.

I was expecting to read something about the temperment as well, since changing the tuning allows for different spacing between notes. This too can somewhat dramatically change the feel of each key.