It would be obvious in the structure of their skull if that were correct. It is a very serious error in cell division. It is noticeable. Very.
Neandertals were also very strong, and very efficient hunters. They were highly social, and cared for their dead. This is one of the reasons why their extinction is a mystery. They had it all: Strong musculature, large brains, and very good imagination.
Some say that they didn't have that much imagination, based on their tools, which didn't change much over a very long period of time. Personally, I think that they were the type that looked at it like "what do I need a ratcheting box wrench for? My socket set works just fine."
Down's Syndrome people are very 'simple'. They are like fully grown, very young children. Quite often, they are just smart enough to get themselves into a serious amount of trouble in a very short amount of time, just like a child can. They can be very difficult to care for. There is no way in the world that they could have existed as a standard in a normal population.
I don't know anybody with Down's Syndrome, but I think that I can understand why your comment was 'hurtful'. Throughout history, children with this affliction have been killed because of it. They're "different". It's not their fault. It was just an untimely error in cell division. Implying that an entire species of extinct people are extinct because they all had Down's Syndrome tends to lend credence to the type who think that their elimination is a good thing.
I don't believe that you feel that way, not at all, especially from your comments. You seemed surprised that somebody would find your comment hurtful. Many abortions are performed today because the parents fear that they may be carrying a Down's baby. A friend of mine almost did so (and she is an avid pro-lifer) only to find out that the test came out positive for Down's because they had the date of conception figured out wrong.
It is something that people who know a Down's Syndrome person, or have one in the family, take very personally.
I believe I mentioned a nephew with Down's Syndrome.