I must say, I agree with him to an extent about his 'greater fool theory' statement.  It's true of bitcoin like no other investment because of its non-physical nature and lack of intrinsic value.  Even gold can be made into jewelry and nice coins.  The only thing I'd argue is that it's money, and we don't generally think of money as a 'greater fool' investment.

Does Mr. Gates not know about Poloniex?  I think he could easily short bitcoin there if he really wanted to.  Hell, with all of his money I'm very certain he could find a way to do it even without that particular exchange.  Now isn't the time to do it, though, in my opinion.  $20k would have been the proper price point, and settling the short at around $7000 would have been appropriate.  I am pretty sure we're on the way back up in a long term uptrend.  Now isn't the right time to be shorting.