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.
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