A recent New York Times article says no. But Dr. Donna Harrison who was at the FDA hearings that approved of requiring the labels for Ella notes that the best of science shows that scientists agree Ella works by preventing implantation:
The European Medical Association technical review articles state that Ella is embryocidal. That means that Ella kills embryos. I attended the FDA Advisory Committee Hearing on approval of Ella, at which data were presented which demonstrated that Ella is around 95 percent effective in preventing a clinically recognized pregnancy. One of the Advisory Committee members repeatedly pointed out to the manufacturers that there was no way the effectiveness of Ella could be explained by delaying ovulation alone.