> it is about what is possible in the language based on how it is implemented (which is C++)

What is possible to express in a language is determined by its specification, not its implementation. Of course, it's up to the implementor to correctly adhere to the spec, but this is also independent of language choice. It's certainly possible, and has been done, to implement a language with GC, TCO, lazy evaluation, purity, etc etc, in a language like C++ which has none of those things. For example, the nix language, which has all of those things except TCO (which it certainly could have in theory) is implemented in C++.