Thank you Mark. You're correct about the geometry, and this is a bit of a pet peeve of mine.
Honestly, the word "coplanar" doesn't have much use in woodworking at all. I'm more likely to use the word "flush" in situations where actual coplanarity is desired.
Certainly in milling we deal with flatness, parallelism, and perpendicularity, not coplanarity. A board with the two faces coplanar would have zero thickness!