Even if it's not actual "soundstage," it's a term that exist for headphones and is used by everyone in the community. That's what the term is Even if it doesn't align with the term used elsewhere.

An example is with display resolutions, it's a ratio, so you should say 16:9 as "16 to 9" but everyone says "16 by 9" which is supposed to be used for actual values and not ratios. But everyone says 16 by 9, so that's now the industry term.

Also soundstage of headphones does change how they sound, it shouldn't be the definitive reason for getting a headphone, but you should consider it when buying.