This result is based on "long-term" being one year of use of THC dominant product. It links to the original study by several US-based researchers.
“Participants who used primarily THC-dominant cannabis reported a greater reduction in PTSD symptom severity over time compared to controls. Cannabis users also showed a greater than two-fold rate of remission from their PTSD diagnosis (defined by no longer meeting criteria for a PTSD diagnosis on the CAPS-5) compared to controls by the 1-year follow-up assessment.”
A summary of the longitudinal study is here from our friends at NORML.