I think a more generous reading of their comment would be: do people with Down's syndrome vary significantly in terms of their Down's syndrome symptoms? Or: do they vary significantly in how they experience their symptoms? I don't have enough experience with Down's syndrome to answer either – I've only met a few such people in passing – but would be interested in knowing the answer.

Edit: I feel I should note that, given the phrasing of the comment, I think your interpretation is closer to the original intent – or at least, a clearer reading of what was said – but I wanted to add this in the interest of taking the strongest interpretation of their comment (and to satisfy my personal curiosity).