That depends exactly on what that sentence means. It is fairly unusual that you need to parse a JSON with unknown structure during any kind of speed-critical section.
My personal criticism is that C++ is usually (not always, though) the wrong language for implementing web-facing applications or APIs. There is a considerable advantage in terms of speed (when using the right framework) but there are so many things that can subtly go wrong in terms of safety and security that you need to be very, very careful when picking C++.
It's fairly unusual that today I don't know the structure of the JSON I'm parsing. It's not at all uncommon to want the server to do the right thing if I add keys to the object which it didn't previously care about, though.
My personal criticism is that C++ is usually (not always, though) the wrong language for implementing web-facing applications or APIs. There is a considerable advantage in terms of speed (when using the right framework) but there are so many things that can subtly go wrong in terms of safety and security that you need to be very, very careful when picking C++.
And I write this as a C++ developer.