Exactly! In it's common use, it is an "umbrella" term, which if you ask me makes it not that useful in communications if you already know your current swing/poly inclinations.
BUT!:
Ya know, there are a few rare couples that use this term literally. They grant each other perfected personal agency. In other words, their respective sex and/or love interests are each others own business. Their commitment to one another is one thing, everything else is another thing. It's not poly, it is more libertarian or libertine than anything I think.
They may share the existence of outside encounters with each other -or not, but it is not expected or required and mostly not done as far as I can tell. We knew two successful couples (both amazing couples BTW, and both 20+years and going last I knew) that did this, even though we could not comprehend how their relationships really worked.
Anyway, if the term "Open Marriage" or "Open Relationship" has an objective and specific meaning, I think it is this more literal one.