True. Here's why-- Plato does write the Allegory of the Cave, using an analogy to explain the true form of reality. Plato uses the allegory of the cave to "show in a figure how far our nature is enlightened or unenlightened." He attempts to enlighten the reader of the true form of reality, but he does this by using the contrast of the prisoners, meaning he attempts to educate the reader about what is real by showing them something that is not real. The shadows are as close to reality as the prisoners get.
Plato elucidates the analogy even further as Socrates and Glaucon discuss what happens if one of the prisoners becomes free:
"he will be unable to see the realities of which in his former state he had seen the shadows; and then conceive some one saying to him, that what he saw before was an illusion."
The freed prisoner struggles to see the clarity of the real world, because all he has ever known as been the shadows on the wall of the cave, an illusion.
Agreed, with this caveat: we are the prisoners, and if we step into the light, we are but in another, vaster cave, and we are prisoners again if we stop then.
Was Plato so limited as to think there were only two layers of "reality" and "illusion"?
Plato is all the greater thinker if Plato is seen as presenting us with a way and an encouragment to take a fresh look at whatever we accept as reality.
The process of looking anew and the knowledge of that process are what make us free.