There are definitely mature, adult, anadromous steelhead that are under 20". In some rivers, mainly south of here, they make up a very significant portion of the run.

Ever heard of half-pounders in the Sacramento?

WDFW uses the 20" rule for exactly the opposite reason than the one posted...they don't expect people to be able to tell the difference between a large rainbow and a small steelhead, so they use 20" so that there is a rule that is useful for fishermen.