The people at Mensa aren't a valid sample of people who score high on IQ tests, because there is a such a strong selection effect for people with certain personality traits, such as wanting to join a club based on your IQ.
"Please accept my resignation, I don't want to belong to any club that would have me as a member".
(Please forgive the flippant response. I believe it cuts to the core of what the parent was intending.)
IQ has precise definition. It's what the tests measure. Intellect has only fuzzy handwavey definition. Correlation between IQ and intellect is about as loose as the definition of the intellect. The way people make the correlation even looser is by defining intellect in even more fuzzy and nebulous manner.