* First of all, having a specification and an ISO standard is good, because it doesn't allow powerful actors like e.g. Google to take the language in whichever direction they want and in a huge ecosystem of compilers, platforms, SoCs, etc gives one a certain amount of certainty on what to expect. But fine, let's talk again in 10 or 20 years and see how non-standardised Rust's doing.
* Second of all, no developer will ever learn the language from the standard document. This is something for compiler writers or developers that need to understand the standardese in very specific cases. A regular developer will learn based on a series of books (such as Josuttis, Meyers and Stroustrup - personally I also like Bancila), then check their assumptions on cppreference! What the experts do is irrelevant for the average developer, because the great majority of us aren't experts in a language, nor do we want to be. It's a tool, not a purpose in life.
* Thirdly, one doesn't have to learn the language comprehensively to be productive, indeed this is not how many developers learn, because they must often learn several languages and therefore satisfice. Yes, there's developers that prefer to go deep, like me, but based on what I read online and have seen in real life, that's the exception. And even I don't go as deep as learning the standard by heart, that's a waste of time when there's so much actually useful domain knowledge to acquire.
That being said, C++ is growing too much and too fast and is too complex. At some point I'd like to be "done" with learning the same language, but Rust and C++ are like a never-ending journey. That's why many developers wisely skip both and why I prefer to invest my time in Python and Go instead of CPP-but-with-memory-safety.