I think it's safe to assume it's legit and acceptable. This is from the Dartmouth Pre-Health Advising website:
PrivateFolio
PrivateFolio is the dossier system we use for supporting letter writers to upload their letters of recommendation. It is also a confidential and permanent home for your letters. PLEASE NOTE: All new medical school applicants must use PrivateFolio. If you have an existing Interfolio account and letters/documents already uploaded, you may continue to use Interfolio.
I used it this past cycle because my school offers free credits through PrivateFolio to students/alum. It worked perfectly and plays well with both AMCAS and AACOMAS. No regrets!