I believe she is wrong, respectfully. Freedom must be defined. Freedom in her context is seen as what others are allowing you to do by edict, law, or force, so it is externally defined; however, I define Freedom by it only being limited by yourself and ability to make decisions internally, so under my definition of Freedom, slavery is only possible if they can prevent you from making decisions.

For instance, say I am under a system of chattel slavery in the ancient world, or in the early 1800s in America, the "slave" was only called that because of the external factors placed upon them; however, slaves could try and escape and could do things they were externally forbidden to do; the question is whether the risk of the consquences would disuade the "slave" from making those decisions. Simply an internal decision which cannot be changed by external means, directly.