By asking the people involved. The website makes clear that the scores are "unverified" but most of them are from the mouths of the people listed. I happen to think they are probably accurate. Allen and Gates are liberals, their scores are exceptionally high. I believe it.
Nobody is claiming that these scores are from the files of the College Board but I happen to think they sound just about right.
A plausible enough explanation, except for one thing:
Why would anyone admit to an SAT score under 900?