I don't understand how you jumped to the membership test instead of literally the .find() method on a string?

The interviewer is not asking to solve a problem here, they're asking for a simple ability to follow instructions, hence the offer to use Google to find the correct answer.

You could make a very solid case for using "in" (it is 2-4x faster), but only after you've solved the task at hand, this is what is expected in interviews. Not knowing the interview meta makes an average Joe basically unhireable in this market.