Ask HN: How Do Puts Work?

I'm reading about Mark Spitznagel and Universa Investments, and in an article they put it as follows: 'his trades, which cost almost nothing to put on during good times, can be sold at almost infinite prices.' Why?

If the price of stock A is now $20, and the put gives you the right to sell at $30, wouldn't the put be worth $10? Or was at nearly infinite prices just an exaggeration? But even then I'm having a hard time understanding the massive returns they've claimed, unless there's lots of leverage involved.