I would point back to indigo's comment regarding fit first. Schools advertise as something, as a kind of short-hand (and marketing tool) for the student that is most appropriate for their specific programming. Find out first whether the programming is suitable to your child, rather than the labels.
On the reliability/validity of measures like the WISC--first a disclosure: I administer and interpret these professionally, so naturally I think what I do is worth doing! As to the instrument--like all psychometric tools, it is imperfect. While the thought is to access underlying reasoning and learning abilities, this access currently must pass through some learned skills, which are affected by environmental and opportunity differences. When tests are normed, developers attempt to minimize these effects by standardizing with a sample representative of the general population, so that environmental factors are captured in the norms as much as possible. (Even children raised in Canada use Canadian norms, and not USA norms, despite the many cultural and linguistic similarities.)
This is one of the reasons tests need clinical interpretation by trained professionals. It's not a number with the precision of, say, a blood test (and even then, there are error bars), but it is the best we have at this time. Someone who understands the kinds of factors that can bend scores up or (more often) down can provide more nuance and context to what IQ scores mean.
[On a side note, certain aspects of the test are more sensitive to learned/environmental factors, mainly the VCI. The VSI and FRI tasks are designed to be novel, so they tend to be less impacted by education, but more by retest/practice effects, especially on the VSI (that would only come up if she had been retested with the same instrument within 24 months, or if someone exposed her to test items prior to the formal evaluation--aka, cheating). Similarly the WMI, but with retest effects probably much shorter term.]
Which is to say, it's pretty good, but not perfect.
But to the question I hear behind this question of test validity: it is highly likely that your child is really as bright as the evaluation indicates. Remember that your DC is very young, and it is unreasonable to compare what they "look like" as an HG+ learner to what highly-accomplished college-educated adults look like. (I have an unusually high number of early college grads in my community, as well, some of whom I have known since they were no more than your DC's age.) And your DC is an individual, on the path to developing (extremely strong) individual gifts in individual ways.
Which returns us to the original takeaway: if this program aligns with who your child is (and she is the same child now as before the number was obtained), then it should be a consideration. Otherwise, not, despite the qualifying number.
