Not entirely true, but the few Mensa meetings I went to were all 'social' ones (met at a restaurant), and I had nothing demonstrable in common with the people there other than having the same biological brain ability. We probably did have some things in common, but no one went out of their way to welcome me, and I didn't bother to try to make myself welcome either, so I quit going.
What I think we may have had in common is some of the same social experiences of being outsiders looking in to 'normal' societal groups. Certainly everyone's experiences are different, but often highly intelligent people will be ostracized (through their own fault or the fault of others or both), and being able to relate based on those shared experiences can be useful in some capacity. The group I went to didn't seem to be in to that.
And you believe this because?
My evidence... I've not seen any evidence to the contrary. We do see evidence that physically messing with brains causing changes in brain function. Brain damage can cause impairment to memory - short term memory is one of the elements of IQ that gets measured in tests that I've taken.
If you have evidence to the contrary that what we measure as IQ is somehow not rooted in biology, please share it.
"everything is physical / material / biological."
I don't believe in ghosts/souls/the immaterial, but this is irrelevant to social conditioning having an objective effect on IQ tests. Yes, the brain that stores information is "physical", but the information is not stored in birth.