I think Boeing has the upper hand no matter how the people vote.
If voted yes, that means there are more people that are not happy with IAM'S recommondation which could lead to a follow on vote to oust IAM from representing them in the future and save 1% dues they pay.(yes, I believe a new vote for no represention by IAM is possible if the majority are unhappy with the "no" recommondation)
If they vote no, the union workers will take the hit and still be unhappy with IAM's representation that allowed Boeing to pull this off, but I'm sure the IAM will file a grievance against Boeing if that is the case
Just my thoughts on what might be going on behind the scenes.
They might be making $15/hour at McDonalds, vs the $12 starting salary at Boeing if a bill in SEATAC is authorized(and has already been passed by the voters). Of course, benefits also have an impact when weighing these options.