Except the law doesn't say that.:;
Their FAQ does:
Off-Road Vehicle (ORVs): If your ORV is required to display an ORV tab, you do not need a Discover Pass for that vehicle. Generally, ORVs are non-street-legal vehicles. You will need a Discover Pass for the street-legal vehicle that transports your ORV to a state recreation site. The trailer does not require a pass. Street-licensed vehicles, such as dual-sport motorcycles or four-wheel-drive vehicles, will be required to display a pass.
A street licensed vehicle isn't REQUIRED to display an ORV tab.
A street licensed dual sport or 4WD is required to display a discover pass.
I see dual tagging a vehicle as a grey area that will be interesting to see if it gets tested by LEOs.
You know the RCW or WAC off hand? I went looking for a couple minutes and couldn't find it.