Not a YC company, but Square was also founded on Ruby. There was a pretty large monolithic Rails app that AFIK, may still exist.

Though we hired a bunch of people from Google, who brought Java, and we went hard towards SOA. Around 2013 there was a Java vs Ruby war in engineering, with many upset people. It ended in peaceful coexistence. But most things in the main payment flow ended up Java.

Would also expect the numbers to even out somewhat if you normalize for the number of startups choosing a language e.g. Ruby

Airbnb and Stripe are both W09 and S09, respectively. Brian Armstrong worked at Airbnb before cofounding Coinbase, which was S12.