C++ and C# share a letter only. They operate in entirely different domains. You can certainly try to write PLC logic or a an IMU Sensor in C#, but that's probably not going to be a fun task.
C++ is used in many non-low-level domains simply because of performance, due to the many kinds of zero-runtime-overhead abstractions it offers. There are e.g. popular game engines written in C++. LLVM is written in C++. Some DBMSes are written in C++. Etc.
C# overlaps C++ in all these “performance-sensitive high-level code” domains.
I'm very familiar with both. My day job in defense is bare metal systems with C & C++ and often integrating them with C#-based test infrastructure.
Is C# perfect for every task, no. But it's probably suitable for most of the tasks that DARPA is targeting to support. The reality is that low-level languages were used in the past because generally our computers were less efficient, and the higher-level languages didn't exist. Most things today written in C++ would probably be written in a higher-level language if designed today.
Infact, I know of a specific case where a vendor is currently in hot water because a GUI tool running on a standard windows machine is not memory safe when in this case some agencies think it should be. This is probably similar to the vast majority of C++ code DARPA is concerned with.
