The problem with your logic is that the reference to "humanoid" in reference to monsters and alignment is based on the game term (as per the game designers and previous discussions on this topic), not the dictionary definition. It's because humanoids (in the D&D sense) are mortal creatures with civilisations and are capable of independent thought. Fey (and other non-humanoid creatures), may be humanoid (in the dictionary sense), but that's irrelevant—they are spiritual beings like fiends, celestials, elementals, and undead whose natures are inherent to their identities.