RCW 9.41.060(9) is just one of several exceptions to the prohibition of concealed carry without a license, carrying loaded in a vehicle without a license, and leaving a firearm in an unlocked car. I don't think that section really falls under "except as otherwise provided by law" in 28B.30.150.
However, I am curious to understand exactly where this aspect of the regulatory authority granted to the boards of regents allegedly comes from. What is the legal interpretation of "full control" of university property, and where do restrictions on that control come from? I mean, it certainly seems to me that the Washington constitution is a pretty strong "except as otherwise provided by law" here - I'm quite certain a board of regents couldn't get away with trying to deny freedom of speech or religion on campus, after all.
Definitely something that's interesting to consider. I'd really like to see that chain of legal logic that starts with RCW 28B.30.150 and ends with "therefore boards of regents have the authority to forbid individuals from possessing firearms on campus," if indeed there is one.