Update: I just purchased a new USB extension cable and replaced the first one I was using. Now the Zmatter dongle is discovered and the Zigbee menu is populated! Yay. It was a bad cable. (There is no room on the back of my Polisy to attach the dongle directly due to the power connector and the USB to my PLM.)
My ISY configuration matches the Wiki instructions (localhost; http; 8080). Admin username and password match what's set in the Admin Console.
I deleted and re-added the ISY on a few prior attempts to start from scratch. Each time the Polisy automatically discovered the ISY (IoP, of course) and the problem persisted.
I think I made progress, though: I deleted the ISY again. This time, however, immediately after deleting it, I manually added it (ISYs > Add New ISY).
The Node Server running on PG3 now shows up in the Admin Console. I think I'm up and running again. Thanks!
dex's post in ZMatter Zigbee not responding on Polisy was marked as the answer