I thought saliva tests actually measured metabolite levels that are present in your salivary excretion, similar to urine tests. Saliva doesn't retain metabolites for as long as urine, but how could cleaning your mouth out cleanse the actual interior of the gland?

I'd imagine that brushing and rinsing like that would stimulate saliva production, leading to a higher likelihood of failure.

Unless I'm wrong (as I haven't researched this) and they do just measure whether stuff is present in your mouth.