What effects could cause an increase of 8 points on a properly normed test across the board? Why would there a significant benefit to being in the control group of this study?

I already gave you three separate explanations for why an increase is possible, even in controls.

your only possible conclusion is incompetence, which isn't evidence which should change your priors. Incompetence is the social equivalent of the null hypothesis, and there is very rarely any significant evidence against it.

I have no idea what you mean by this, and I think that if one accepts their incompetence, the best thing to do is to ignore their data as having been poisoned in unknown ways - maliciousness, ideology, and stupidity often being difficult to tell apart.

Assuming only incompetence as you have, the expected result would be equally erratic for all students.

Why is that? The competent result is, since IQ interventions almost universally fail (our prior for any result like 'we increased IQ by 8 points' ought to be very low, as in, well below 1%, because hundreds of interventions have failed to pan out and 8 points is astounding and practically on the level of iodization) and the followups confirm that there is only a much much smaller effect, that there is no or a small effect. Any incompetence is going to lead to an extreme result. Like what they found.

As you say, it's been confirmed by other studies.

'Confirmed'? Well, this is an active debate as to what counts as a replication. Near the same magnitude or just having the same sign? If someone publishes a study claiming to find a weight loss drug that will drop 100 pounds, and exhaustive replications find that the true estimate is actually 1 pound, has the original claim been "confirmed"? After all, both estimates are non-zero and both estimates have the same sign...