Their point might be that the ability to read, has a non-zero influence on how intelligence as well knowledge; both of which are what IQ tests try to measure. The two signal values are related to an unknown degree.
And it shouldn't really. Assuming dyslexia is the result of structural differences in how the brain operates and not simply that a person is behind the curve on reading ability.
If say dyslexia behind the scenes looked something like the difference between being left handed or right handed, a persons natural athletic skill (IQ) might improve their odds of throwing a good right handed pass yet regardless of athletic skill their outcomes would be better throwing left handed.
If say dyslexia behind the scenes looked something like the difference between being left handed or right handed, a persons natural athletic skill (IQ) might improve their odds of throwing a good right handed pass yet regardless of athletic skill their outcomes would be better throwing left handed.