That's total BS. I gare-own-tee it.
http://onlinelibrary.wiley.com/doi/10.1111/j.1556-4029.2011.02021.x/abstract
This article from Forensic Sciences is interesting. They had street samples of weed with concentrations of THC ranging from 15 to 22% THC, about the potency of hashish. The concentrations were high enough that they investigated the DNA of the plant material to see if it had been genetically engineered. They had some historical numbers from a decade or so ago showing much lower levels.
I can’t post the article, but I can refer to the abstract. There are lots of hits online about it, I searched for “journal of forensic sciences genetically modified cannabis” and got many hits. I’ve seen a few other articles in DEA’s “Microgram” journal about increased potency, too.
The journal article does suggest an increase in potency, which in and of itself is not bad, it just means that people use less mass to get the same effect.
http://www.huffingtonpost.co.uk/dr-raj-persaud/has-cannabis-been-secretly-modified-_b_1688684.html
Here is a Huffpo article that is a bit less scientific, but it does mention some researchers and papers to do some further reading.
Oh and the JofForens article found no obvious evidence of genetic manipulations.