Had another look at the clip. "Karaoke Klowns! Fo'
Shame!" But if you look at the frames around 1:25, those strings look distinctly black (
and nylon) against those frets. That's a pretty strong indication though it's not a sharp enough glimpse, at least on the youtube clip, to tell definitively. Certainly
sounds correct - could be rotosound, or la bella maybe, or any number of other brands? - There's a distinct edge there for you, I'd reckon, by the look of it. Nate's got a good point regarding the board too, which is potentially not an unsubstantial input - they could easily hammer down any undesirable brightness there.
And, like Harry, I
do hear some, very tasteful, compression in there. That
could be happening at the board
or from a pedal. I wouldn't be surprised if it was the latter, at least in part. Magic
[ed] Had yet
another look

Hee Hee… If you look at the frames around 4:16, when the house lights are up, those strings don't look so dark. So, flats maybe instead, as has been said ^