Facebook - Broadcasting live video to millions (code.facebook.com)

I'm curious why Facebook was willing to sacrifice delivering adaptive stream to get better latency. I guess they explain that for "Live for People," it makes more sense to have less latency through RTMP, as opposed to possibly worse video or connection. However, I know Periscope is using HLS for all of their stream, save for Android where it looks like RTSP.

Anyone have any thoughts or insights on Facebook's decision to use RTMP as opposed to HLS?