Interesting comment from the interviewer about interviewing 500 DC being interviewed, they have expanded on last year. We always found KGS staff very open and I do not see what reason they would have to misrepresent numbers.
Just to clarify, in case you may have mis-read or mis-understood my message - I just referred to the fact that I did not know anyone with a general interview that got a no ("We do not know anyone with a general interview that got a no - from a sample of almost 20"). I made no statement at all on only general interviews getting an offer (to be honest, your reference to it was the first time that I heard that 'myth') - I have no personal experience or knowledge on the matter, I would very much assume that DCs with subject interviews get offers, otherwise the school will not see a point of interviewing them.
Best of luck for you and your DC with the KGS process (and any others for the matter). It is a great school and one that we grew very fond of during last year's process (and probably the one that we/our DD found most difficult to reject).