Sure anyone can, but OSI is the organization that popularized the term in the first place and its definition is widely followed, so there's no need to make such point in this particular case.
That’s no different to saying “You can define black as white.” That’s true but nobody is going to pay attention to you if you do.
The analogy falls flat because the words that make "open source" both already have meanings that make the OSI definition confusing to say the least.
The common way it is understood should probably be named "libre source" or something, because it means more than that the source is just out in the open.