Nobody has ever claimed that Vocablearning is the most significant factor affecting the performance of a student. The scheme was only put forward as a feasible and cost-effective means to help the students to tackle their vocabulary need in their study.
Scientifically, there is no way to prove that there is no other more significant factor affecting the postulation of a given hypothesis. If the query is a valid statement to invalidate a hypothesis or a theory, no one single valid hypothesis or theory can exist in the world. The data of the project already disproved the hypothesis that Vocablearning has no or adverse effect on the learning of students.
If we need to investigate the effect of any factors not considered in a given hypothesis, a separate investigation is required. By scientific convention, the given hypothesis is still valid as long as it is not disproved.