Technology

Yeah but it's fairly simple.

You can generate Steam keys using the Steam developer tools. This allows a game key to be purchased on any storefront that supports selling them, which can then be activated on Steam.

The main requirement? You can't price those steam keys on a 3rd party store cheaper than on Steam itself.

For that, it means if the 3rd party store takes a smaller cut than Steam itself would take, the developer makes a bit more profit through almost no additional effort. Steam is the system users use to download and update the game, and cloud save syncing, and community guides, forums, workshop, etc.

The developer is, afaik, more than welcome to also sell a UPlay key if they partner with Ubisoft at any price point they want (regardless of the Steam price) because Ubisoft is the taking on the burden of distribution, etc.

The only price requirement Valve imposes is on selling Steam keys on 3rd party storefronts. Not UPlay keys. Not Xbox keys. Not Epic Store keys.

Edit: and I read the article, while albeit short (can't access the linked Bloomberg article sadly), they claim exactly that, that the version on UPlay was significantly cheaper than the version on Steam for essentially the same game. Valve was arguing that Rainbow Six Siege needs to change their pricing on UPlay or they would be delisted.