Rush wasn't reporting "facts"--he was giving his opinion. If Rush had backed up his opinion with facts (citing sports reporters and their commentary as proof) as he did on his radio show the week following the incident, he might not have gotten into as much hot water. But they were gunning for him, so it probably wouldn't have made much difference.
Remember he basically went unchallenged on this issue that day, and was more challenged as to how did the guy get the "Ring", and in the end all parties agreed on that issue...McNabb was a "good investment".
But you are right, it was an opinion and Rush presented it as such. Walker is the one calling it "fact".
I thought he only had a limited time to talk on that show, so I guess he didn't think about having to bring up specific incidents. Why didn't the other guys on the show ASK him about it? Were they so scared of the subject that they just didn't want to continue it? Kinda makes Rush's point doesn't it?