Still trying to pick apart this guy's tone.
At first I figured muted flats on a P - nice. The quick decay and generally "vintage" sound seemed like a pretty classic flats on a p vibe, of which I'm very familiar. The more I listen though, it seems there's there's just too much mid bite/presence for these to be flats. However, there's really none of the deep metallic grind of rounds. Also closeups indicate there is no colored thread wrapping the wound part of the strings, pointing to flats.
Could also be groundwound or halfwound. Any thoughts?