Actually, that is completely true. The potency has more than doubled since 1983.
Sources:
Mehmedic Z, et al (2010) Potency Trends of r9‐THC and Other Cannabinoids in Confiscated Cannabis Preparations from
1993 to 2008. Journal of Forensic Science, 55(5):1209‐17.
University of Mississippi, National Center for Natural Products Research, Potency Monitoring Project Quarterly Report 107 (January 2010)