mtDNA does not show this to be the case.
The days of paleontologists and physical anthropologists doing their Neanderthal studies only on morphology are drawing to a close.
Morphology is a limited tool, and the mtDNA evidence that is starting to accumulate is eclipsing it quickly.
To me this is both sad and encouraging. I did a lot of morphometrics in grad school, using multivariate stats. I used these techniques on skeletal populations during the last 6,000 years, but others have been using them on fossil specimens, including Neanderthal.
With the advent of mtDNA studies they are now going to a new level of analysis. As much as I love morphometrics, the critical Neanderthal questions are now being answered using DNA.