Ety Mology

From a blog commentator at the NYT via Michael Quinion:

Bed bugs is TWO words - not one. The general rule for writing out common names of insects is as follows. If the insect name is a misnomer (e.g., the dragonfly is NOT a fly and neither is a damselfly), then the whole name is written as one word. If it is not a misnomer, then it is written as two words (e.g., house fly, which is a real fly). The bed bug is a "true" bug and therefore is two words.

Quinion appends a long discussion suggesting that this is really and truly real and true,  however obscure.  Does it mean that "bedpost" should also be written as one word because not  true post?  Or "firesale" as one because you are not really selling fire?