Intelligence and stupidity are both complex things with multiple causes. A "general factor" of intelligence or stupidity doesn't take away the fact that some people are particularly good or bad at particular things for particular reasons.

Incidentally, it may be worth mentioning that the people whose work I A Richards is referring to here, who exhibited such "failure to grasp the meaning" of (in this case) some not especially obscure poetry, were undergraduate students of English at the University of Cambridge. So we're talking about relative stupidity here; these are people selected for intelligence and with at least some interest in (and apparent aptitude for) the material. Some of their errors really are pretty stupid, though.

[EDITED to fix an inconsequential typo.]