I bought it here for the Mac version but also have Windows and use Steam - but it says there are no keys available and to try back later. Not sure what the plans are for Steam and cross-platform going forward now that it's officially on steam.

I'm in the same situation. The store page certainly makes it look like we'll still get Steam keys.

My Steam key was available today when I tried claiming it again so it just seems like there is a temporary delay in key availability. Hoozah!