At first I thought margined abdomen referred to the appearance of a vertical groove right down the middle of the abdomen. Now I am thinking that it means That the yellow bands are broken, or narrowed at the middle of the abdomen.

Also, the smaller posterior end of the thorax in this specimen is the same color, more or less, as the rest of the thorax. In other specimen it is a yellow color, closer to that of the yellow stripes. I'm not sure whether yellow is dull, but to me it is more dull than the metallic color seen here.

Am I on the right track, or have I interpreted this incorrectly?

Well, I'll try to clarify - maybe one of the experts can provide a better explanation. Margined abdomen is difficult to see from the top (dorsal view); it is best seen from the side (lateral view) especially when the shot is unobstructed by wings. Margined abdomen refers to the black margin at the side of the abdomen as shown on this Eupeodes which clearly shows that the yellow bands do not extend to the underside. Whereas on this image of Syrphus the yellow bands continue to the side and there is no black margin. The other part that you mention as "the smaller posterior end of the thorax", I believe that you are referring to the scutellum. A Glossary may be helpful.

I am not 100% sure on any insect and I doubt that I ever will be. You have contributed many images of Syrphid flies here and elsewhere so I appreciate and value your opinion even if its only 99.9% accurate. :-)