True, but note that, in contrast with Mensa, the Triple Nine Society continued to accept scores on tests taken up through 2005, though with a higher cutoff (of 1520) than on pre-1995 tests (1450).

Also, SAT scores in 2004 were found to have a correlation of about .8 with a battery of IQ tests, which I believe is on par with the correlations IQ tests have with each other. So the SAT really does seem to be an IQ test (and an extremely well-normed one at that if you consider their sample size, though perhaps not as highly g-loaded as the best, like Raven's).

But yeah, if you want to have high confidence in a score, probably taking additional tests would be the best bet. Here's a list of high-ceiling tests, though I don't know if any of them are particularly well-normed or validated.