In western states with a lot of rural places only serviced by interstate they sometimes never passed blanket prohibition against non-motorized traffic on the interstate property.
I'd bet I-5 in Seattle is bicycles prohibited, but there's no blanket prohibition of bicycles on freeways. Like you said, there's a lot of parts of Washington where the freeway is the only reasonable road, so you can't prohibit bicycles or pedestrians.
Bikes that have motors and don't fit the e-bike tiers are motorcycles ... and could potentially be legal to operate on a bicycle prohibited freeway, but they'd need to be registered and it looks like these don't have plates, and the operator would need a DOT motorcycle helmet which was not present, and I'd bet these are also missing out on the lights and stuff you need too.