The reason the Cornish are not mentioned is that when the quote came about Cornwall was considered by the rest of England (biased words, I know) to be just another county and not the Kingdom it had been prior to Alfred the Great. There are other counties that could claim to have been Kingdoms at the same time but there is no such sense of separateness as has grown in Cornwall over the last few years. This may be due to the lack of a separate language or the fact that few English counties are out on the edge like Cornwall.