Hello,
nice to meet you :) .
@Truly Open Sourced?
No, the source code is open source > https://9nations.de/feedback/help/ or directly https://bitbucket.org/trutzimt/9nations/src but at the moment some graphics has a proper license. They will be chance in the future.
@Mods
What kind of mod you are interested in?
At the moment their is no real mod support, but you can checkout the code/gradle build, change it, and check in it again.
If you have any wish, feature request, idea, feedback, ... i also love to hear it :)
See you soon