To a certain extent, they aren't. The same compilers and tools are used for both, many developers mix the two in one codebase, and use small features from C++

To a certain extent, they aren't. The same compilers and tools are used for both, many developers mix the two in one codebase, and use small features from C++ in "C programs" because the compiler allows them to. For example before recent C standards, function overloading by argument types or declaring variables in the middle of a function. But Python2 and Python3 are way more similar than C and C++.