It certainly appears to be. That's a bit confusing, however. Every example photograph rather prominently notes the camera, not phone, it was taken with. The features seem primarily useful for camera-and-raw-processing photographers. The website doesn't seem to indicate that it's iPhone only, but the feedback section, and the lack of any way to log in or sign up other than the link to the iOS App Store, certainly suggests that it is. The feedback section even suggests that it doesn't support iPads well, and is phone-focused.

What's the intended usage here? Take a photograph with your camera, transfer it to your computer, process it there, export it... then transfer it to your phone in order to post it? Why?

Pretty common for mobile apps to start on iOS only.