I for one, would not think that abstractions are "not real". They exist as they are, with the qualities that they have. I think perceiving the world in "3D" is dependent upon abstractions. Do we see in 3D? Or are we just pretty good at making a rough guess at it? Is not the assumption that there is an "other side" to any object an abstraction if it is not directly witnessed?

I think they are just saying that an abstraction is not a concrete entity. We make that distinction all the time in object oriented programming. A class named Dog represents (is a blueprint for) a concrete entity; a class named Animal represents an abstract entity. There are animals, but that's a higher-level abstraction of all non-plant, non-virus, non-prion living things.

Sometimes we make a class called Configuration or File or Node. You can't point to those kinds of things and say, for example, "there's a configuration lying on the table, go get it for me, will you?". A configuration is a concept, not a literal thing. A class representing a file is normally a stand-in for any file, it's a collection of behaviors associated with files generally.

Perhaps the notion is that they do not exist as a true representation of that which is abstracted? That's about the best I can do.

Because something exists as a thought doesn't mean it exists out in the world. I can think all kinds of things that may or may not be true.