The statement that all of us are purportedly able to coherently conceive or imagine a certain situation—for instance, an imitation man or a zombie—is rather trivial from a philosophical point of view because ultimately it is just an empirical claim about the history of the brain and its functional architecture. It is a statement about a world that is phenomenally possible for human beings. It is not a statement about the modal strength of the relationship between physical and phenomenal properties; logical possibility (or necessity) is not implied by phenomenological possibility (or necessity). From the simple fact that beings like ourselves are able to phenomenally simulate a certain apparently possible world, it does not follow that a consistent or even only an empirically plausible description of this world exists.
—Thomas Metzinger
In fewer words: we can imagine things that cannot exist.
In even fewer words: we can imagine the illogical.
What a fun game: Impossibilities are imaginable.
I think he’s saying something more limiting—we cannot tell if we imagine things that cannot exist.
or even as far as—we cannot tell if things cannot exist. :)