The article puts forth a couple paltry reasonings. First, there's a shortage of tooling. Second, there's no existing standards for collection/analysis/etc. Neither of these attack the real problems that immediately spring to mind and are the basis for having no standards or tooling. What if you have little to no mucus available? What if the mucus is unknowingly tainted? (as it's for filtering and catching things)
Blood makes much more sense because it's a relatively well regulated and pure substance. Anything that got in your body and is circulating, isn't an external contaminant.