It's in development in WebKit, it has been for a long while (the author just noticed it now and went with the full-of-assumptions article for the ad money) and it means absolutely nothing. Yeah, it will probably (and hopefully) one day end up in Safari. On the other hand, it also wouldn't be the first time WebKit had a feature that Apple chose to disable, especially on iOS Safari.
Apple has traditionally locked down mobile Safari to a far greater extent than desktop Safari when it feels there is a compelling UX or security reason. For instance non-fullscreen video is not possible on mobile Safari, but easily doable in desktop Safari and native applications. I'd expect this to be similar at least initially - it could still come to mobile Safari some day, I just think the initial release will be for desktop.
While I understand there's an argument to make on standards for them supporting it in mobile Safari, a mobile web browser is really a lousy place to build software like this. Customers are going to be much happier using a native application. We could certainly get into a discussion on what Google is doing with the mobile web on Android that would make mobile Safari support more important, but Apple is not going to be moving away from installable Swift/Objective-C based applications for the foreseeable future, and that's going to remain the best implementation route for a while.
There are really only two viable options, both plugins. The first is Flash, but the problem here is that you need a Flash media server on the back end. Twilio Client release branch 1.2 implements this. The second is the Temasys plugin which allows you to run WebRTC as you normally would in Chrome. Neither are good options to be honest.
I wouldn't assume that. Since Safari Technology Preview is going to be updated every 2 weeks, as soon as WebRTC is stable enough, it'll be included in the Preview.
I'd be surprised if WebRTC isn't on the list to be a major feature of Safari in this fall's OS X 10.12 release, which will be previewed at June's WWDC.