C++ is somewhat of an outlier due to the sheer size of the spec(and getting bigger all the time). Not everyone will use the same subset of features either, which complicates things.
That said, aren't you using C++ as a proxy for other needed skills though? Many Python programmers are not exposed to concepts like vtables, or they do not know how a linker works(or why it is even needed). Heck, many won't have done manual memory allocation or manipulated pointers.
Even though most people won’t use everything, all c++ programmers will need to understand RAII, manual memory management, lifetimes, effects of inheritance and some* undefined behaviour. We’d probably take people with C experience, and maybe Some D or Rust, but really for a mid level C++ job we wouldn’t accept much less than C.
Funny that you mention linkers, as they’re a bit of a minefield. Lots of the behaviour of linkers is platform (and implementation) dependent, I.e. are you building a shared library or an exe on windows or linux, and there are different gotchas for all of them. We wouldn’t expect people to know al of those things, but it definitely helps when it comes up, and you don’t have the same problem (ODR violations come to mibd) in many other languages...
I like your way of putting it, but I think those concepts (RAII etc) are fairly easily learned by a competent developer in any language. Most languages have analogous concepts if you use them long enough.
A shop that says it requires years of experience in a particular language is probably making their own hiring situation more difficult than they need to.
C++ for example is definitely a sharp language that takes some time to adjust to, and experience in C will help.
However: I think any competent programmer with a few years of experience would be able to gain basic proficiency in a month or two and probably daily-level-mastery in 6-12 months.
I say this as someone who had 10+ years in Java and only passive experience in C. I've managed to become mostly proficient in C++14 in about a month of full-time learning. After 10+ years of programming, I can pretty quickly adapt to the C++ way of thinking about things. I don't think I'm alone in this style of learning a language, and many hiring teams seem to agree.
That said, aren't you using C++ as a proxy for other needed skills though? Many Python programmers are not exposed to concepts like vtables, or they do not know how a linker works(or why it is even needed). Heck, many won't have done manual memory allocation or manipulated pointers.