If ever there were a situation where that argument didn't apply, surely this is it. This is a multi-billion project, in the defence sector, for a major US ally

If ever there were a situation where that argument didn't apply, surely this is it. This is a multi-billion project, in the defence sector, for a major US ally. So for one thing, it seems very likely that someone involved in the carrier project does have access to all the relevant Windows source and build infrastructure, one way or another. Relevant Microsoft staff have probably been checked as well. In any case, obviously you can't install some OSS and then just "hire some programmers" to fix bugs in this environment. Using Windows on naval vessels is not unprecedented. The US Navy "Windows for Warships" situation showed that it is already done, and sometimes problems are then found in trials, and those problems do then have to be fixed.