I don't think the advice applies for C++ either. The last time I worked with a C++ code base I tried to understand some std:: API semantics by reading the code but I failed every single time. I'm not an expert C++ programmer by any means but I think I've passed the novice stage where you try to learn the language. Maybe for C++ constant learning of the base language is needed :-)
For Go and C on the other hand I think it works very well since the core languages are so simple it's viable to read others code successfully without being a language expert. I read the Go stdlib code all the time if something is unclear in the documentation (the docs are good but sometimes there are edge cases that are not clearly documented).
Let me cite a single line from the MSVC implementation of the C++ standard library (https://github.com/microsoft/STL/blob/main/stl/inc/xtree):
Also, good luck understanding this class member definition. Hope you figured out _Scary_val! (Hint: Empty base class optimization.)