They are both correct. We call this set comparison.
1. He is one of the more prolific writers of our time.
2. He is one of the most prolific writers of our time.
"More prolific writers" suggests a subset of writers whose output is above a (unspecified) mean output. And "most prolific writers" suggests a subset of writers whose output is above an even higher mean output.
For example, those writers whose lifetime works number, say, 1,000 may be considered 'more prolific' than those whose output is only 250. Higher up the scale, those writers whose works number, say, 5,000 may be considered even more prolific, i.e. the 'most prolific'.
Yes, I understand your explanation. I have been wondering about this for quite a while because I learned the rule that when you are speaking of more than two, the word, "most" is correct.
So many people use the word, "more" and I wonder how I missed that during my school years.