I hear you, RWS, and that's a fair enough thought; I still feel it's insufficient. Instinct -is- a kind of logic; moreso in the ork's case than anyone elses. There's no reason that it should be harder to predict that an Eldar Autarch will say "We must destroy the human forge world of Clavian 9. With it, their arms supply will dry up - and with it, their river of success."... than to predict Warboss Necksnappa should say "We's goin' to the humie mech world down there to take all them gunz an' gubbinz and wotnotz'!"
In order for Imotehk's "illogical foe" hypothesis to be valid, an ork WAAAUGH! would be just as likely to plow into an isolates shine-moon staffed by two priests and nine gilt servitor-slaves as they would into its neighbour, the forge world previously mentioned.
To be frank, Orks are pictured primarily as making up for their intellectual deficiencies with numbers and enthusiasm, but never as a random, illogical force in the universe. I call bull.
Additional, the Stormlord is portrayed both as a Strategic genius in the macro-scale and a small engagement paragon. The fluff story listed for him is a step-by-step dismantling of his foes in individual engagements, particularly of the farseer I've previously ragged on.