My guess (I haven't had a chance to fully digest the paper) would be that they are saying spam is designed to emulate high quality signals so discounting it in

My guess (I haven't had a chance to fully digest the paper) would be that they are saying spam is designed to emulate high quality signals so discounting it in the quality ranking could mute actual organic quality signals. Instead the approach they suggest (from my quick read) is to not include any information in the quality ranking system about spam but rather allow the spam classifier/filter to deal with it on it's own terms.