This is true of work you do on company equipment during work hours. If things were as you describe, no one would ever be able to do open source, let alone sign standard FOSS CLAs or the like.
Don't over-generalize, both of the big tech companies I've worked at in the last 10 years have this clause and require _explicit_ specific approval for any outside work, open source or not. Otherwise, yes, according to the contract it is their property. The enforceability of this debatable, but it is most definitely something that happens.
