The other day there was an article about the importance of motivation in intelligence and success. From my understanding of what this article is, that's a glaring hole in the theory.
At 5, children will differ hugely in how motivated they are to do well on an IQ test, and parental influence may well play a large role.
A 5 year old won't exhibit much working memory if they're not concentrating on the test, and if they're not test-motivated by 5, they're less likely to be so when they're older.