The article links to IMCO. IMCO have a PDF. The claim appears to come from that PDF. They are saying that Mexico grows a different strain. I don't know why Mexico couldn't grow different cannabis.
May have something to do with the climatic requirements of different varieties of the plant, similar to how Gewürztraminer performs best in a cooler climate but Grenache needs a hot one.
(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.