On average they reject 98%. Mensa's cut off is the top 2% of IQs
These two statements are not congruent unless the distribution of IQs in people applying to Mensa is equal to the ditribution of IQs in the general population. I doubt that to be the case.
But this does not mean that they reject 98% of the people that take the tests. People interested in taking that test must already be, I suppose, in the top 10% of the IQ distribution (125+?), or so they think. If this is true they they do reject 98% of the applicants but only 80%. Yet I suspect they reject even less.
Any source or third-party evaluation of their acceptance rate?