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.
The big problem is just Seattle being incredibly underpoliced and really no laws under homicide are really enforced or punished.
