I've certainly implemented it, but there's no way I have it memorized, it's not an obvious algorithm at all. I'd be flabbergasted to be asked to implement it from memory with no warning. It seems clear to me that one would be testing for if the person happened to have looked at the algorithm recently, not if they were competent.
If you don't remember this, it's OK. If I was the interviewer I'd still give you the description above and see if you could implement it. Most candidates cannot.
I think expecting recent CS graduates to implement a basic sorting algorithm or binary search, etc. (say, in a garbage collected language they know well) is reasonable, for people with actual work experience I think it makes less sense and the focus should be on more recent experience if relevant to the job.
I was referring to any sort of "memorization" - just talking and watching someone code should make it clear if they just recently read an article about an algorithm, or if they actually understand it.