That sounds like a reasonable read, but I (the poster who posted about learning resources) haven't posted on this thread before, so I'm not the author of the prior post(s) to which you're referring.
I do know that, for the most part, if a child's academic profile isn't strong enough to get into a STA the Beauvoir guidance director will usually let them know pretty clearly. Sometimes these people seem to persist with applications, but not too often -- and sometimes these people are just deaf to any "bad news."
The reason why kids don't get into STA can be pretty mysterious... but it really seems to be a confluence of factors that play into it. If a kid has a mediocre academic profile, has a tough visit, doesn't have stellar reports, then that can do it. Any one of those things doesn't typically exclude a kid from admission. STA's also reluctant to accept kids with low WISC scores (low, in relative terms, being 110 or so or lower). All the Beauvoir kids submit their WISC scores, by the way. BUT PLEASE NOTE! There's this rumor that STA won't accept kids with a WISC score below 115 or 110 or whatever. This is patently, absolutely untrue. There's also this rumor that they won't accept kids with math or reading ERB scores below the 5th stanine (in terms of independent school norms). Again, ABSOLUTELY untrue.
And believe me... I'm not doing myself any favors by stating such so flatly since I'm an educational consultant and tutor who benefits from parents' anxiety.