That's not necessarily true. Companies certainly like to advocate for the idea that they automatically own any work you do that's relevant to any of their business interests, even if that work isn't relevant to your particular job. But that doesn't mean it'll hold up in court. Just a couple of days ago I was reading something about how courts regularly side with employees instead of employers in disputes like this, how they tend to interpret that as only applying to work relevant to the employee's job and not to other parts of the company. Unfortunately I don't remember where I was reading that, so I can't go find it again.

Of course, take this with a massive grain of salt. I'd certainly prefer not to have to go to court to try and figure out if this is actually true.