Hi and thank you for messaging. :)
It's because of the 6th clue: "Neither squares nor sculptures are blue." So the medium 'sculpture' cannot have the form 'square'.
that phrase reads to me as; the sculpture is not blue and the square is not blue.
!((sq = bl) | (sc = bl))
=>
(Sq != bl) & (sc != bl)
You could say instead
There are no square or blue sculptures
!((sc = sq) | (sc = bl))
=>
(sc != sq) & (sc != bl)
See
https://math.stackexchange.com/questions/25091/translating-neither-nor-into-a-ma...
I agree, it is a bit misleading.
I pretty much derived the clue description from here: https://logic.puzzlebaron.com/how-to-solve-a-logic-puzzle.php?t=neither-nor-clue...
I see if I can come up with a clue that is more clear. I think "There are no square or blue sculptures" is missing the hint, that squares also cannot be blue. ;)
The complete clue would be something like: "Neither squares nor sculptures are blue" AND "Sculptures are not square".