It's already been mentioned in a few comments, but I imagine a big part of this purchase was influenced by the SEO power of the domain. CSS-Trick is a crazy powerful domain [1]
Google loves old domains with authority, and still, to this day, it's a lot easier to rank a site built on an aged/expired domain than it is on a fresh domain.
Buying powerful domains on auction sites has shot through the roof in the last couple of years. Here's a couple of example on Godaddy (Godaddy auctions tend to have the most powerful domains SEO-wise) https://www.godaddy.com/domain-auctions/gutenberg-net-414405... https://uk.godaddy.com/domain-auctions/freewebtemplates-com-...
I imagine they will eventually 301 the domain to the main DigitalOcean domain.