Ok technically on a molecular level, sure. But don't let the perfect be the enemy of the good. At the end of the day, more of the denser gas will be in contact with the wine than oxygen, reducing the rate of oxydation. In fact, I'd hypothesize that the O2-wine contact in a narrow space ("topped up" carboy) that has regular-pressure air in those remaining couple inches might be more than the O2-wine contact in a wider surface area that was first purged with argon then reduced to vacuum.
The use of denser-than-oxygen blanketing gasses is a commonly-accepted effective practice not just by commercial wineries but endorsed by many reputable resources I've found, such as:
https://winemakermag.com/technique/1308-inert-gases-techniques
So I'm a bit hesitant to dismiss it simply because there'll "always be some mixing of some molecules to a degree".