In the US, your employer owns any intellectual property you create that happens as a result of your "regular duties". This has been settled in court many times here that "regular duties" extends to creative projects done outside of work hours and not on company time - assuming you were hired to do creative work like he was. If you're hired by Ubisoft to "create video games", then if you create a video game of your own while working there (even at home), Ubisoft owns the rights to it - or at least can usually successfully argue in court that they should.
This sort of thing is typically also covered under employment contracts and company policies that have to be signed and agreed to get jobs at big companies the size of Ubisoft.
All that is to say - since they're in the EU I imagine a lot of the dystopian copyright / trademark / patent laws like these don't exist there, and they're likely fine. But as an American it's weird to me hearing someone admit to starting a project like this while working for a big corporation.