On the one hand, they're only allowing the brighter students into the program in the first place:
Classes, capped at 25 students, often have waiting lists. This year’s kindergarten class tested 110 students for the program, taking only 50, said Judy Rivera, who runs Channing’s program.
Then they try to state that the students are bright because of the dual-language:
“We have statistics here at Channing that the dual language students are consistently scoring above their peers...”
Now I may not be the brightest bulb, but doesn't it stand to reason that if the best and the brightest are being admitted to the program in the first place that, regardless of the additional language, they'd still be out-scoring their peers afterward?
In other words, one has nothing to do with the other.
I suspect the reason for the waiting list is not quite what you think.
Dual-language programs work best when there is about a 50-50 balance between native speakers of each language. If the school is heavily minority, almost all english speakers who apply will be admitted into the program, but there will be more selection among the spanish speakers.
Based on actual results, both groups do better than their academic peers in other types of programs. Even in seemingly unrelated subjects such as math.
Your line of reasoning may be valid for the spanish-speakers, but it does not explain the performance of the english-speakers.