The last sentence is the key. Those who choose birthright already are at least somewhat predisposed toward Jewish identity. To have a bit more valuable study one would have to interview them and measure them before birthright, and then test them much later and see to what extent they have affiliated with judaism and the Jewish people vs a control group.
Thousands gather in Kuala Lumpur for rally urging Malaysian leader to quit (AP)
from the article: More Birthright grads are marrying in, says new study