Sometimes tech interviews will ask questions that are more along the lines of API memorization. They can be valid questions too, I've worked with "senior" developers that would implement their own sort algorithms (in .net) rather than use the built in ones. "Does this enormous framework I'm using have a way to do x" is a question some developers never ask.

It all comes down to the question asked, if they were literally asked to randomize a list then OP gave the correct answer, if they were asked to implement a list randomizer they were wrong. If the interviewee didn't make this explicit they were wrong.