Your citation is wrong. The definition of "agricultural land" is found at §030(2):
(2) "Agricultural land" means land primarily devoted to the commercial
production of horticultural, viticultural, floricultural, dairy, apiary, vegetable, or
animal products or of berries, grain, hay, straw, turf, seed, Christmas trees not
subject to the excise tax imposed by *RCW 84.33.100 through 84.33.140, finfish
in upland hatcheries, or livestock, and that has long-term commercial significance
for agricultural production.

You'll notice that all of the illustrative uses involve actual production (as in, produced on / by the land), not industrial processing operations. The reference to "RCW 84.33.100 through 84.33.140" further supports my interpretation. Those statutes set forth the valuation of forestland, not saw mills. Calling feed lots agriculture makes no more sense than calling  a tannery, slaughterhouse, vegetable freezing plant or apple packing plant agriculture.