I finally found a decent replay of it (
NFL Video Galleries, about 2:30 in, thanks to
In Which We Attempt to Understand Why Heyer Was Flagged on Thomas' TD | Mr. Irrelevant), and watching it again, Portis hit Dockett, then Heyer no more than 1 second later hit him, with Portis watching on. Leads me to believe even more that it was a questionable call...