Huh, that doesn’t match my understanding (TBF I don’t work in the field).

Looking again at the study… Dr. Jain had a second person split the students into two groups. I’m not sure whether she conducted the followup exams or had yet another person do that. Assuming that the students were strict about not talking about which toothbrush they were using, I suppose you could call it a ‘blinded’ experiment. I don’t really see how you could double it, though.