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?

We hold all objects in our mind: the objects we perceive outside our self, as well as our thoughts. Some are objective are independent and some are subjective,dependent notions. Some thoughts phenomenal represent an existent and these appear to be independent of us, others have only virtual existence and they are dependent on us such as abstractions. Abstractions don't have a phenomenal existence, we can only perceive particulars.

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.