"since I'm using the Steam client, I can't purchase it."
Completely not true and a common mis judgement...
What it means by 'Keys purchased here can not be used to activate DCS products on Steam!' is that it can't be activated on steam to get the product on steam. Eg. if you bought Black Shark 2 on the website, you can't activate the steam version of it.
However, that put aside, if you purchase it from the DCS website, what you can do is download the files from the website. Install it to the steam folder and still use your code. It will all still work, trust me. The only difference is you have to manually update it.
I have both the BAE Hawk and the FW 190 from the DCS website but I still use the steam version of the game. It works perfectly but I forget to update them :)
Hope this helps and if you have any questions, just let me know.
TL;DR No news on steam but it is possible to use the version you buy on the website with steam, I've done it.
