True. For instance there's knowledge that can't be directly perceived by any one individual, given variations in our sensory apparatus. Even among so called "normal" sighted people, the sensation of red varies from person to person. I have a normal sighted colleague, and there are "red" LED wavelengths that seem very bright to me, but he can barely see them. The color matching equipment is partly based on an accommodation of those variations.

>Being unable to perceive color and color relationships does not equate to being unable to have knowledge of color and color relationships.

Yeah, I think that was their point. Whatever 'knowledge' we think we have with qualia is actually something that, functionally, we seem to be able to get along without and do just fine. Which makes you wonder what functional purpose we don't have by not having 'qualia'.