With coding in particular, there is major confounding due to the need for sensory motor control (handwriting) and good visual processing & visual memory. Strengths or weakness in both these areas will also impact some of the subtests that make up the GAI (block design for example) but others may not be impacted at all.
Using the two subtests you mention is a particularly good example because a blind person with no hands could score 18 on similarities just as much as any other person might be able to, and would be completely unable to undertake the coding subtest at all, despite possibly having quite excellent executive function.
