There’s nothing inherently bad about taking the MCAT in September–so long as one is not applying to medical school that same year.
Taking the MCAT in September and applying the NEXT year is a perfectly fine strategy.
It takes 4 weeks to get a MCAT score back and med schools close their application windows in mid-to-late October. Plus once the score is back, the applicant still has to get through AMCAS verification (which takes about 2 weeks) and write & return a half dozen essays (secondary applications) for each school. Even if one somehow manages to squeak in under the deadline, there are very, very, very few interview slots left. (Med schools start interviewing in August.)