Does concrete sealant slow the growth of moss?
In a way it does. It's a lot like Rain-X makes water bead up on glass, masonry sealant will make the water tend to bead up and run off of brick and concrete. However, much like Rain-X, it wears out every once in a while -- probably more often in the PNW where it'll get wet all the time. 

The moss grows because there's easy moisture to be found within and on top of the brick. However, the sealant won't prevent moisture that's deposited on the brick by fog / mist or dew from collecting -- all three things are common in the PNW due to the relatively high humidity. Therefore, it'll help a bit, but it's not a total or even ideal solution. 

Although -- the Thompson's sealant is not the best on the market by far. Look at the Behr stuff.