I didn't investigate deeply; I'd never heard of haproxy prior to Fitbit. Someone from site ops told me they use nginx + haproxy but it would be easier for me to just set up nginx.
SSL support has been around for a while, but 1.5 has been considered beta for a few years now, though that hasn't stopped it from being deployed in a lot of production environments.
Before SSL was rolled into haproxy, nginx was often a good candidate to handle the SSL termination. Stunnel is also common, and stud was popular for a while, but seems it was abandoned once haproxy could handle the job.
This article does sound to me like SSL support in haproxy is brand new: http://seanmcgary.com/posts/using-sslhttps-with-haproxy