Hi Ruben, thanks for your interest!
What we're building with MoodleNet is very different to a PHP-based learning platform such as Moodle. As a result, we need to make choices that allow us to build something that's going to work well and at scale.
If you're interested in the reasons behind the choice of Elixir for part of the MoodleNet project, please have a look at this lengthy explanation by Mayel de Borniol, our Technical Architect here.
In addition, check out the diagram below. You'll notice that there's many different components to MoodleNet, and code for which we're using Elixir is just one part.
When people and organisations set up their own version of MoodleNet, it's likely that they'll only ever need/want to modify the front-end parts. For those, we're planning to use ReactJS. We haven't made a final decision about the Core API-as-a-service, but the chances are we'll be using PHP for that.