Probably this player had $20K which he could afford to lose, I know it sounds big to you and me and most people here, but rich people are gambling too and this is probably nothing to them.
What you told is correct as this bet is in btc that is why it make difference because of when the bet was made that time btc price was very low and if you calculate in usd then that much of bet as usually made by big gamblers so it wont have affected him, but thinking of betting with this much high bet you wont have guts to bet it. and even the gambling site also wont support that much high bets.