The drums have a touch of swing to my ears, like 52 or 53%, just enough to unbuckle the belt a notch.
The way I hear it, the synth is actually pushing the groove a little (i.e., is ahead of the beat) and has zero swing, so it creates tension.
MC is laying back, lagging gently behind the beat with a healthy slur and mellow flow. That's what relaxes the whole vibe, creating a nice pocket that releases the constant tension that's hammering away and rounds out the corners of everything that's happening behind it.
Stack those up and you got the groove. Balance and compress everything just right and you bring it out, make it pop.
Mostly, though, it's the MC. You could drop that vocal over a dozen different beats and it'd still groove. You could drop a stiff MC over that beat and it'd be an annoying brick of tension.
There's no groove like a human groove.
Gregory Scott - ubk