If I had to interpret that...I think Richard is referring more to subdirs of sites he's already listing...not just the domain itself.
ie..
say you had
http://www.mysite.com/
listed...and on that main site you had a link to
http://www.mysite.com/gallery1/images.html
Richard doesn't then want you submitting
http://www.mysite.com/gallery1/images.html
as well.
IE every site on a domain should have fresh content on it and be exclusive of each other
Short form:
multiple sites on the same domain are allowable as long as they don't interact.
At least that's my interpretation and how I submit my sites to RR which seem to make it past the review process no problem.