> My kids have a photographic memory of each of those special pieces and which set they came from.

Read the whole sentence- this is clearly an informal use of 'photographic memory' to indicate that his kids are really into Lego and keep track of details in the way only kids can.

> Of each of these special pieces

I didn't see him making any generalized claim. I recall the things I'm really into better than I do things I'm not, which is all I think he was saying about his kids. (My four year old remembers facts about volcanos - including the name of the scientist who made the claim - waaay better than he does where he left his shoes, so it tracks for me!)