The open source part of this is nearly useless, if not completely so, as evidenced by the fact that Firefox won't be using the source code. The "you don't have to pay the MPEG-LA" part only applies to the binary module. If you want to pick up this code and ship it in your own software, you'll still need to pay the MPEG-LA.
The reason they are doing this is to put more weight behind H264 in the political battle in the IETF over the MTI video codec for WebRTC. But if they succeed in making H264 the MTI, and you want to write a mobile app that interops with WebRTC for doing some form of realtime video, you'll won't be able to use their source code without paying the MPEG-LA. You'd have to use their binary module, downloaded from their servers, which, as they mention in the Q&A, is impossible on iOS.
The announcement seems to try hard to smudge this distinction and push it into the fine print, but there's still a patent "elephant in the room".
TL;DR: This doesn't solve the patent issues surrounding h264, and doesn't make it anymore suitable as an MTI for WebRTC.