Intelligence =/= Game Knowledge
Same as if i ask you to name the 14th, 18th and 24th presidents and their place of birth. You not knowing does not mean you are stupid, it means you did not have that knowledge. Intelligence is much more akin to your ability to gain knowledge or to infer and discern things based upon your current knowledge.
If the Rammus knew that you build MR for Ap and Armor for AD, but chose to build as you stated then yes he is stupid.
If the Rammus just was unaware that the MR he'd build or the AP damage was more important than trying to buy items to give himself attack damage (And thus use his passive) then perhaps he is lacking knowledge...or...oh shit...no, he is retarded. Lol
EDIT: also, wtf did you give them Azir, Anivia and blitz lol