Not allowing to use C++ cause C is a subset and I should know C? Fintech 🤦🏻‍♂️

Recently I had a phone screen at a fintech company and the interviewer pasted a stub code in C, and did not let me modify it to C++. He said I can use the standard libraries from C++, but rest of the things, like allocating char buffer and lists should be done using malloc and i should use char** to allocate a list of char* buffers. Even upon offering that I can make his stub into C++, he said he doesn't agree that C and C++ is different and when I say I know C++, I should by default be knowing C. Then after a couple of segmentation faults due to converting my set of strings into his char** buffer, I was told to give up. I want to understand from others if C++ and C are same in their understanding. Just because C complies in C++, does it mean i should brush up my C skills too? In my experience, I have never been in a phone screen where I was forced to use C instead of C++. Is this reasonable to expect in other interviews as well? Or should I assume that most tech engineers do know the difference between these two languages. My experience with other large fintech companies have been similar. They ask out of book university level definitions the entire interview and then would give something like implement shared pointer or virtual class and functions. It seems the engineers there focus more on "if this person knows the definitions". Advice to those applying at fintech - brush up on your semester 1 undergrad computer course. Most questions will be directly out of book, looking for proper definitions. After giving these interviews (and clearing a few of them), I no longer want to work in fintech as a SWE. Engineering quality seems so rote memory focused.