I would check for sure on that with McMurray. I know there is a white Cornish variety, but I have been unable to find anyone but McMurray who offers anything that even sounds like it.
My copy of "Storey's Illustrated Guide to Poultry Breeds" shows a White Cornish as an example of the Cornish breed.
The reason I never pursued the White Cornish question with McMurray is they did not have availability on them until June anyway.
From what I read on their website though, I would have to think they are true Cornish, and not hybrids. I would call and confirm this though.
My sister raised Cornish Roasters from M Mcm a couple years ago and they did not look like broiler birds at all. They got very big and were extremely healthy. They were not the obese genetic freaks we are used to seeing with regular broiler birds. They looked like Cornish.