I have not ever heard of anyone actually call the Coast Range 'mountains', so you are 'correct' from that stand point. Technically, they are defined as mountains, hence the reference to them as a 'range'. I'll settle for calling them 'big hills', but there is at least one 'hill' there that is referred to as a 'peak' (as in like Pike's peak), which I think of as a mountain. That would be Mary's peak. Relatively speaking in relation to the Cascades and Rocky's, the Coast Range might amount to a "'HILL' of beans."
The idea of the "Willamette Valley" also implies it is defined by geological features on two sides.