My understanding was that this is part of their on-plane server that you connect to via their wifi connection (for free). I think Southwest capitalized on a bet by skipping all the of the terrible touch-screen based back-of-seat entertainment systems and simply host a bunch of content and live TV on the plane that anyone with a device can connect to. People already carry around all the devices they need as they said.
To quote from the article, users will use their own devices:
>To use Beats Music free on aircraft, users will have to have a device compatible with the Beats app, which includes iOS and Android, and can also work with any web browser that works with the browser-based Beats streaming site.