Not surprised Linux was the choice here, nor surprised by the remains of the FUD by MS still around. First of all there is kernel space and user space and the software to the Northrop Grumman MQ-8 is most likely in user space, and that is theirs and only theirs, be that in the drone or on the ground or both. For instance if you have, say a router, probably running Linux then you will find in the instructions a text telling you where to find the kernel source or just the version number of the kernel. There was a time when some big companies, like Cisco tried to cheat but those things seem to be OK to day.
And also, I can tweak a Linux kernel to my hearts delight, it's only if I start to deliver it as a product that I have to show my tweaks to the kernel. As far as I understand Google is not forced to revel what they do to the kernel as they use it only for themselves (GPL2).
I used Solaris and other *nix versions like HP-UX, Aix, SCO, For-Pro for some 15 years. All acceptable, all sightly different but good. No Linux then but that has changed in favor of Linux a lot since then.
The thing to remember here is that there is more power behind the development of Linux than behind any other *nix version, with a question mark for iOS.
Somebody pointed out that when you compile a Linux kernel you simply leave out all the stuff your kernel does not need, that, of course, is an advantage with a compilable kernel. I did it once just for fun, but the reason I mention this is that there is still a bit left of the FUD that you have to do it, you don't.
The real time question mentioned in previous comments is very interesting too, lots of that on the web to read about. Traditionally *nix and real time are mutually exclusive. *nix systems have a "democratic" scheduler not very good at interrupts regarding processes. (and I know there are those who can explain this better). Anyway, things have changed here too. If you look at the traditional real time providers they have, more or less, all moved to Linux. It is possible to deal with the real time demands on top of the Linux kernel, as some do, and real time features have been added to the kernel for many years, faster processors have also helped.
The only thing that would surprised me is, if I actually knew how largely Linux is used to day. Its use is growing all the time, and why not, it's just a fact.