> I was once told that the top universities actually communicate to each other which applicants they accept
This is true though the contacts are not formalized and are idiosyncratic. It's more about finding someone you know on their admissions committee.
> in order to avoid sending admissions to the same top candidates
This is false. Top departments compete with each other on everything, and there is no good mechanism to enforce this purported commitment.
The SP affair gives me new respect for Christians. It is an unlikely story superficially if you don't know the process, but I have seen the person for myself and know of several people at admitting departments that have made a decision. But to the agnostic, the threatened, or the outright afraid, this might as well just be ranting about a bearded mystic I saw at a seafood shack.