The issue with spleen and black bile in humoralism isn't that it overproduces it, which it doesn't, but rather that it overaccumulates it.

>“The gall, placed in the concave of the liver, extracts choler to it: the spleen, melancholy; which is situate on the left side, over against the liver, a spongy matter, that draws this black choler to it by a secret virtue [emphasis mine], and feeds upon it, conveying the rest to the bottom of the stomach, to stir up appetite, or else to the guts as excrement.”

~ Robert Burton, Anatomy of Melancholy