Yes that is definitely a violation all the sings really do is deter people from open carrying who don't know any better. Kitsap county where I am still has a few of theses sings up ever after the county announced they would be removing them a few years ago. I recently saw a few still posted at island lake in silverdale and when I asked they parks department why they had not been removed I was told because they didn't have a big enough budget to pay anyone to go around and remove all of them.
RCW 9.41.290 State preemption.
The state of Washington hereby fully occupies and preempts the entire field of firearms regulation within the boundaries of the state, including the registration, licensing, possession, purchase, sale, acquisition, transfer, discharge, and transportation of firearms, or any other element relating to firearms or parts thereof, including ammunition and reloader components. Cities, towns, and counties or other municipalities may enact only those laws and ordinances relating to firearms that are specifically authorized by state law, as in RCW 9.41.300, and are consistent with this chapter. Such local ordinances shall have the same penalty as provided for by state law. Local laws and ordinances that are inconsistent with, more restrictive than, or exceed the requirements of state law shall not be enacted and are preempted and repealed, regardless of the nature of the code, charter, or home rule status of such city, town, county, or municipality.