This is no surprise. They've been partnering with ("hiring") players who have a large following on social platforms (YouTube, Twitter, Twitch) for the past year or so just to acquire their likeness. It's no different than Google buying start-ups for no reason other than acquiring the employees. They're clearly using social platforms and their potential for what is essentially free advertising after the initial investment to their advantage.
Twitch has public API, the effort (arguably minimal) required to plug in to it through the client is negligible when you consider the fact that it's free advertising.
How do you figure it'll be shitty performance? If anything, cutting out the middleman has the potential to increase performance. Rather than having a "one size fits all" type program, the client can pull the data itself and feed it to Twitch in whatever way suits RuneScape specifically. No need for messing with codecs and compression, it'll be tailor made. Considering they've partnered with Twitch streamers specifically, they have that experience on hand and should have no problem putting it to use.
OT: I always use OBS to stream and I think thats the best. When Twitch integorated with Minecraft, It took more CPU tov stream from Minecraft than OBS itself