It's UDK so there is kind of modding support. Whatever modding UDK supports (which is not much) like maps, and models basically. Just build it with identical UDK source and based on their page I'm guessing they will cook it for you or something.
It's also using a ton of slightly modified UT content. I'm not sure whether that is allowed by the UDK license or not. That flying ship is identical to a Raptor, it just has longer wings. That ground speeder is identical to a Viper but the body mold is slightly different.
To me it looks uniquely geared towards attracting people who were following UT and lost some features that they wanted. It's a freaking trailer, even UT3 looked good in trailers. They also have no dedicated servers. Likely to die.
Don't get me wrong, I'd be fine if they do well. I might even try it out. But the video doesn't give me much hope for it.