> Of course, I am comparing individuals having a medical condition (dementia) with, as far as I understand, cognitively intact study participants, but I think it doesn't meaningfully weaken my argument.

This is exactly why.

Because dementia has specific non-global effects on cognitive ability. It's a disease. The impairments caused by dementia aren't a linear projection down from a non-diseased state. Even if outcomes in terms of how the study defined them end up in a similar place, dementia isn't actually supporting evidence in any meaningful way.