The article is slightly wrong. They opened with $19.53. It's currently hovering around $18.
First trade was at 18 dollars and that's roughly where it ended. But the IPO price was 15 which does mean it is up 20% from the IPO price.
Robinhood reported 19.53 as the opening price. I noticed Google also reported it as 18.
Opening price is different from the IPO strike price. Institutional investors and rich folks already bought those shares before they rang the bell for the market.