The Austrian military didn’t just adopt LibreOffice; they actually contributed back to it. Over five person-years of development work went into adding features they needed. Those improvements are now available to everyone using LibreOffice, which is pretty cool.
I’d assume they would probably hire a few developers to get the things they specifically need into the software.
My fear is they will decide, at some point, to toss the GPL in the toilet because of “national security.”