Non-duality means not two.
“Not two” doesn’t mean “one.” The problem with the idea of one is that one is a thing. Even if you are imagining a vast oneness of all you can still see it in your imagination. You can think about it. You can get fixated on it.
What we’re trying to do is get rid of the fixation. Free our minds.
Also “not two” just simply doesn’t make any sense. That also is the point.
Maybe that’s the main point.