Nope. This is, at best, a misconception. At worst, it’s an intentional misinterpretation. They have a term that prohibits the sale of Steam keys cheaper elsewhere. Game publishers are welcome to put their game up for sale on other sites for cheaper; They just can’t sell Steam keys cheaper. Basically, Steam wants to protect their own product keys from being undercut.
Ubisoft has their own storefront, and their own launcher. If you buy games on the Ubisoft Store, you get access to them via Ubisoft’s launcher, called Ubisoft Connect. Ubisoft is free to sell their games at whatever price they want on the Ubisoft Store, as long as they’re not granting access to the game as a Steam key. If you buy it on the Ubi Store and get access via Ubi Connect, then everything is fine. The only way it would be a breach of contract is if Ubi ran a sale on the Ubi Store, then gave players access via Steam. If you buy it cheaper than Steam on the Ubi Store, you won’t get a Steam key.
You can even sell DRM-free versions of your game for cheaper. As long as you’re not selling Steam keys, you’re fine.