To qualify for Mensa you have to be in the top 2% of the IQ spectrum. So taking into account that people can practice IQ tests etc etc, that top 2% bracket is still fluid and ever changing. If more people were scoring higher through practice, the top 2% score would be higher and harder to attain.
There have been numerous studies on cultural bias of IQ tests which have been proved and disproved but are too long to get in to on here. The bottom line is that they are a good guide to raw ability. The point is though that it doesn't really matter what raw ability you have - its how you use it that counts!
That 33% of Cambridge students can't get into Mensa is to their credit, not their detriment. They are clearly fulfilling their potential and making the most of what they have - unlike someone at a redbrick with a genius IQ who is lazy or hasn't found effective ways to study.
The key to genius is not just having a high IQ, it is doing significant things in the real world along with that. So by that measure, yes, IQ is a flawed scale.
FYI I qualified for Mensa but didn't join as it is elitist, snobbish, and mostly a sort of self-affirmation society for people whose only thing to brag about is being a member of Mensa!