I don't think I quite agree with the power/responsibility analogy. To me, "power" the way its used in the quote implies some kind of control over something/someone else. That's not really the kind of relationship I see between a programming language and the software that uses it - if I write a program in C++ I don't really view the C++ committee as having "power" over my program. In addition, it's not something a spec really does anything to address - you can be plenty irresponsible with your (hypothetical) power with or without a spec
I'm not sure I'd agree that a programming language has a duty to produce a spec either, whether it's for foundational software or not. Outside of legal/regulatory requirements, I think a hypothetical "perfect" low-level/systems language would be used no matter whether it had a spec or not simply because it was the best tool for the job. In that sense the language devs' "responsibility" would simply to make the language as suitable as possible for users' software. Of course, legal/regulatory requirements throw a wrench into that thought experiment, but hopefully the point made it across.
None of that is to say that asking for a spec is unreasonable, especially if you're required to have one to use the language. I'm just more on the skeptical side as to its overall importance.