My experience as someone who has a windows pc and a small and light Mac for travel, is that you can see directly from the steam page if it is cross platform - it will show both a windows and OS X symbol. In that case I can buy the game and install it on my pc and my Mac. If it shows the windows icon only, it means there is no Mac version deployed on steam.
As all the recent combat mission titles on steam only seem to be on the windows platform, I would guess when you send keys to Slitherine the resulting steam keys deploy only the windows version (irrespective of whether the original battlefront keys were OS X or windows). As a person with both platforms, I like being able to install it on either platform, so I asked the question. If indeed steam is windows only, I’ll buy it anyway as I have a windows platform. But it’s a nice to have - and some people may assume they will be able to access a Mac version on steam, so probably good to be sure.