I couldn't get that link to open so went off an image search looking for what might have been it. I'm hardly about to argue with your knowledge of contemporary Sikhism, but the iconography you get by typing "rabab of mardana" into a Google Image search gives you very inconsistent results.

This guy is playing an Afghan rubab and the Wolverhampton gurdwara isn't complaining:

http://www.sikhchic.com/article-detail.php?cat=4&id=66

Whereas older pictures usually depict something much closer to a sarod, with this display blissfully ignoring the fact that it's got two different instruments there:

http://www.tribuneindia.com/2002/20020413/windows/site.htm

The overall shape of Mardana's instrument in the old pictures ranges from a short sarod to a tanpura, with playing positions all over the place.