That might because they are targeting Embedded C++, which explicitly eschews these features.
And IMO, the final Embedded C++ spec is such a terrible idea that it doesn't absolve them. Having written a C++14 RTOS, if I had to give up multiple inheritance, namespaces, sane casts, and templates, I would have just written the damn thing in C to start off with. The value proposition of institutionalized C with Classes doesn't do it for me.
See https://www.youtube.com/watch?v=TYqbgvHfxjM for what you need to do to have C level perf while using the type system to still get you something.
> The header only nature of Kvasir and very sophisticated use of the volatile keyword
This isn't a particularly encouraging sign…