So because open(2) is implemented in C in the standard library, I should only call it via C, and never invoke the wrapper function open() in python or whatnot?

“Hybrid” does not mean what you think it does. The platform may be built on code in many languages, but to program for that platform, you only have to know one.

An application running inside a browser VM is anything but native, hence integrating it with the host OS turns it into an hybrid solution.

Using your example, C is native, Python is not.