I think it means ego is just a label for a thing in the world.
What exist itself is the truth.
Not the label.
The call for a try to define ego as a precise as possible ("as a mathematician") and in the sibling post from user blueprint probably shall make you attemt just this, but fail at it. I think the "ego" dies when you see what you are looking for in what it is. Not in words not in labels not in labels. Just become aware that your model-maker is trying to model the world that really is there. You are what you are beyond the modeling process.
What exist itself is the truth
How could you describe it? How could you speak of it? Doing so requires modeling it and therefore it is not the real thing anymore. Therefore you can not really speak about "it" without just referring to a experienced experience.
No one can speak of something they haven't seen as it is.
Trying to thinking about the "real" world behind and trying to grasp and feel and see it all at once as it is gives me funny tingly feeling in my stomach. But thats is probably not what the they mean.
One fun thing to consider: When there are no models no labels no boundaries no judgements, where does something start and where does it end? Does that even make sense? Objectively the real world is just what it is, one. No things at all. These are just models. The underlying world is still what it is. One.