It's finite upside vs infinite downside, and he knows.
Like Nassim Taleb (and any proper thinking mathematician), I would think the downside is limited (Bitcoin can't become less than $0) and the upside is unlimited (or: potentially much higher than the downside). The perfect example of an asymmetric bet that Taleb has written extensively about.
See the concept of an asymmetric bet explained further here:
http://contrarianinvestorsjournal.com/?tag=asymmetric-pay-off#The basic idea behind the asymmetric payoff strategy is simple. First, you structure your bet in the market such that if you lose the bet, your loss is very tiny, but if you win, your gain is very massive. Next, you bet that the market will crash within a specific period of time. If you lose that bet, place another bet for the next period of time. You do this repeatedly until the day of the Black Swan event when your profit overwhelmingly overshadows your accumulated small losses.
Obviously, the disadvantage of this strategy is that it requires fortitude to absorb small losses indefinitely while waiting for a highly rewarding final vindication in the end.
Buffet, wrong since 2014:
https://www.forbes.com/sites/kashmirhill/2014/03/26/warren-buffett-says-bitcoin-is-a-mirage-why-marc-andreessen-thinks-hes-wrong/#2c391e827f3c