I don't understand this part of the article.
Producing high-quality consumable marijuana is a bunch of work. You have to be careful to separate the female plants, prune low THC content leaves and stems from the flowering buds, etc; the cartels don't seem to put this much work into the cannabis they grow.
Cartel marijuana often has tons of leaves, stems, and seeds in it. It's earned the name "Mexican ditch weed."
Most people today do not go to their dealer and ask for the kinds of cannabis they have. They just take whatever is there.
Forced reliance on word-of-mouth advertising is very important as well in avoiding the market for lemons problem, but I don't think Akerlof considered that.
(http://imco.org.mx/images/pdf/reporte-tecnico-legalizacion-m...)
They're using THC content as a weak metric to compare pricing. They admit the weaknesses of this approach.
And "less THC" isn't necessarily a bad thing. People still buy plenty of lager even though vodka and gin have a lot more alcohol.