I know W. Lloyd-Thomas and Clifford L.B. Hubbard did some of the older research on the corgi breeds (1930s).  W. Lloyd-Thomas mentioned the Teckel connection (he called it "Tekal") and dated the breed's orgin to around 1200 BC in papers he wrote (some of which I've read).  As is typical in writings of that era, his history is flowery and wordy and doesn't give specifics about the research.  He does say that mid-Cardiganshire is "particularly wealthy in the remains of early Celtic fortifications (source: "What the Modern Corgi Owes to its Cardigan Ancestors," AKC Gazette, October 1935)," and I can only guess that it's from these remains that the corgi has been dated.  Incidently, it might be better to call the early corgi a "livestock dog" rather than a "herding dog," as it was more of a guard...not unlike the Rottweiler.  I'm curious what the reason for the doubt is, though...does the corgi just seem recent?  I see no particular reason for skepticism...Celts have been around for a long time.