Thanks for the fast reply!
This option returns the Delaunay ROI based on the Delaunay diagram, which is a bit different from the Voronoi diagram. It does unfortunately not work on the Voronoi diagram.
However I just found an alternate solution involving the plugin “voronoi” in Process>Binary.
First use the multipoint tool to select a few points on a duplicate of the image, then:
run("Convert to Mask");
run("Analyze Particles...", "clear add in_situ");