"Shut down 3 or 4 unicorns" - wonder who they were - and I find it a bit funny that he doesn't seem to know the exact number. Or maybe he's just trying to be less specific on purpose.
Valuation is very much a vanity. You can inflate it in a variety of ways. By the time the companies get to him, I think the joke is that you can't tell whether it was actually a unicorn or not. If common stock is worth 0, nobody cares anyways.
Unicorns do not exist, but there are a billion suckers out there ready to trade something for less than that something might really be worth.