Ada and C++ were the only realistic options at the time, and Ada developers are difficult to hire.

But honestly, with this sort of programming the language distinctions matter less. As the guide shows you restrict yourself to a subset of the language where distinctions between languages aren't as meaningful. Basically everything runs out of statically allocated global variables and arrays. Don't have to worry about fragmentation and garbage collection if there's no allocation at all. Basically remove any source of variability in execution possible.

So really you could do this in any c style language that gives you control over the memory layout.