Also, there are many sorts of intelligence
Yes obviously there are. But these Y6 tests - whether taken for indies or grammars, and yes no distinction between them - test reasoning and verbal ability. In other words, a particular kind of academic intelligence and potential. So other types of intelligence are a bit beside the particular point.
In any event, the ISEB tests are very similar to grammar school tests and can be tutored and prepared extensively for. So they don’t really say anything about any child’s intelligence. An average child could do really well if extensively prepared and vice versa
Oh goodness Araminta. You don't seem to have been standing where I have over the past couple of decades, watching cohorts of parents despair when their extensively prepped kid doesn't get in to the chosen school :) And I have to say you're quite wrong about indicating intelligence, if by that you mean academic ability/ potential. That's the point of them and that's exactly why very bright and experienced top school HTs (and others in less selective schools, but with a much lower pass mark) continue to use them, just as they've done for years and years (although they've been adjusted to make the best attempt possible to mitigate the effects of prepping and tutoring). I'm not sure you have the same sort of experience as top school HTs in judging the fitness for purpose of these tests but the consensus in the educational world is that they're the best tool currently available and that they do a good job. If you do, my apologies.
Why would it matter if a child in the rf didn't pass the Eton test? It doesn't make that child a less valuable person.