Just remember whether you participate in STEM or not - it does not impact your chances for admission. Part of the purpose of STEM is to expose people to USNA, so it is not a direct reflection on the competitiveness of your application package.
I have no idea if the vacancies have been filled or not for this summer, just don't read too much into it either way.
I know that I've posted multiple times but I am getting worried any new stem acceptances.