Photo editing is bitmap editing, and Pixelmator is well positioned to implement the non-destructive property.

By virtue of being OSX-only, they can abstract out the trickiness of RAW conversion (something Adobe must heavily invest in by virtue of being cross-platform), and rely on Apple's very good implementations instead. (side note: Apple is sometimes faster to implement RAW demosaicing support than Adobe itself).

This simplifies the problem to purely that of bitmap editing. It's important to note that unlike Lightroom, Pixelmator is already hardware accelerated from top to bottom, this allows real-time rendering of the entire edit stack on top of the unaltered original data, potentially giving us much more responsive feedback as users (sliding a slider and having it respond with much less lag than Lightroom, for example).