The methodology used for confirmation tests is the GC/MS which is NOT an assay, per se. Its apparent that the article is loosely tossing around the word "assay" like a baseball in a bullpen.

While immunoassays will react to THCCOOH and other THC metabolites to varying degrees, the confirmation assays are designed to detect only this metabolite."
...which is basically the same thing Ive stated above - minus the loosely (and incorrectly applied) "assay" reference.