You are incorrect. (Well, you're correct that Spamhaus doesn't filter content -- but they don't filter anything, they publish lists that various filtering software uses.)

http://www.spamhaus.org/faq/section/Spamhaus%20SBL#270

I hear the SBL can also block domains, how? What is "URIBL_SBL"?

Yes, the SBL can also be used as a URI Blocklist and is particularly effective in this role. In tests, over 60% of spam was found to contain URIs (links to web sites) whose webserver IPs were listed on the SBL. SpamAssassin, for example, includes a feature called URIBL_SBL for this purpose. The technique involves resolving the URI's domain to and IP address and checking that against the SBL zone.

And of course they also have the DBL (Domain Block List), though I don't know if that existed back when PG ran into problems.

Do you have a link to the false positive rankings? I'm curious as to how that is measured.