It's from a related stream mishap involving a very loud restaurant ad.
edit:
Elon confirmed the center core landing failed catastrophically due to a
propellant deficit engine ignition failure on the landing burn. The droneship took some damage too (if I heard correctly, audio was bad for the initial statement).
edit #2:
Better audio quality quote than the initial statement re: the core's fate in response to a question from Bill Harwood from CBS (bless him):
https://youtu.be/sytrrdOPYzA?t=17m13s
We hit the water at about .. 300 mph .. roughly 500 km/h .. heheh, so that's hard.
Um, and.. about a hundred meters away from the ship.
Which was enough to take out two [of the ship's] thrusters and shower the deck with shrapnel ..heheheh.