My guess is that this is to do with the versioning that takes place for gateway/quiz type assignments. Because the ability to take multiple versions of the assignment is built in, handling what assignment is being graded (or worked) has to be managed slightly differently than for a regular set, and I suspect that this is not included for gateway/quiz sets in the code supporting essay grading on the statistics page.
So the short answer is: I expect that your observation that it doesn't work is (unfortunately) correct. The fix is probably to go into the Statistics content generator and update the grading code to check for gateway/quiz assignments and to pull the appropriate usersetversion assignment for those. The difficulty will be that there are multiple possible versions per student, so the code will have to decide how to display them. It might be that one displays only the last one taken, or all of them if there are multiple.