The interviewer was right. If you don't know "if substring in string" by memory, your python programming is substandard. This should be automatic for anyone who works with python as a primary language. But if you can't even Google to find "if substring in string", why would anyone pay you even minimum wage to be a programmer?
I can't tell if you're trolling or not, but this is a ridiculous thing either wya.
I've used Python as my main language for ~10 years in various professional roles (DS, DE, SWE) and I so rarely need the exact construction `substring in string` that I probably would have blanked on it too in an interview. 99% of my string processing is .startswith/.endswith and re.search, that's just the way it goes. Hell, I know the difference between re.search and re.match by heart (do you? no? you're substandard!) but I genuinely forgot that `in` works on strings.