Why is "Tio estas pulvora kafo" wrong?
Because that would describe "powdery coffee" and the subject of Duo's sentence is the powder, rather than the coffee. Put another way, "coffee" is an adjective describing the type of powder in this sentence. "Tio estas kafa pulvoro" works.
Exactly the sort of explanation I was looking for. Dankon!
Shouldn't be. Whole sentence is odd; "coffee powder" isn't really a thing in English. Ground, maybe.
It's probably referring to instant coffee.