Based on this MIT press release stating that controlling for socio-economic factors test score was still significant in predicting outcome in college. You want to challenge that you need to come up with data.
18 years of growth differences cannot be made up by "exposure to things" from age 18-22. It helps, but it does not resolve it. If someone is truly smart they will score well on the not-very-difficult ACT/SAT without any test prep in the first place. These are stupidly easy tests by international standards. Just suck it up and recognize that at the high level that these schools operate at, if you are below a certain score range, you are not ready, regardless of how many "opportunities" you are given.
That's not to say that some people aren't capable in other ways and will do just fine in life, but these schools are not for them.
Based on?? If someone is smart, but denied opportunity, often times this can be resolved by exposure to things - even for an 18 year old.
Indeed, environmental factors become less important for intelligence starting precisely at this age.