While they don't specifically mention it in the post, they probably had to put some effort in making sure ALPN is still supported [1]. It is in their case, which means they either run a custom compiled version of Nginx (compiled against either OpenSSL 1.0.2 or LibreSSL) or have a system-wide up-to-date version of OpenSSL.

Either way, nice move Dropbox!

[1] https://ma.ttias.be/day-google-chrome-disabled-http2-nearly-...