Marko has been around for over a decade at this point and powers most of eBay. It's not the oldest or the largest, but it's got a pretty solid track record
Ryan was working on Solid before he joined eBay/Marko. Both projects have benefited from the shared knowledge and approaching a similar solution space from different angles.
He eventually got the opportunity to work on Solid in a more full-time capacity and decided to take it, but still talks with the Marko team from time to time