When I first heard about this, I also thought this. And maybe it really does only apply if you give away a Steam key with the purchase.
Unfortunately, the only example I have on top of my mind did give away a Steam key. But the drm free version and the Steam version share save game spot and are compatible and have the same content. Unless one wants to include Steam achievments as "content".
Would be cool, if you share your answer, should you find any precise answer to that topic. A thread concerning this price parity rule pops up once in a while. And in general I see price matching even on pages that do not offer a Steam key with the purchase, so I assume it either is not tied to the Steam key, or there are other reasons to match.