Yeah, hence "supposedly". The club's criteria is based on some form of largely immutable or hard to change personal trait. I can understand this in one way: as a support group. Highly intelligent people don't always have easy lives, especially when growing up. But if it was a support group, you'd have very little use for a test.
IQ is very much so not genetically determined. See for instance the flynn effect which is happening far too fast to be the result of genetics. IQ is only ~50% heritable.