I use the word "unintentional" in this case where Ayn Rand employed two opposing ideas in different contexts. The first context was the prepared and edited portion of Introduction to Objectivist Epistemology (2nd ed.), the second context was the impromptu Q&A found at the end of the second edition.
On page 52, Rand wrote, 'Aristotle regarded "essence" as metaphysical; Objectivism regards it as epistemological.'
However, on page 230-31 she wrote, '"Metaphysically" here refers to that characteristic which, in the nature of the thing, makes other characteristics possible. "Epistemologically" means the way we would proceed to discover a causal explanation. You see, I could not put the second into a metaphysical category, because nature as such doesn't explain. A thing just is, and certain characteristics are the causes of other characteristics. But metaphysically—that is, apart from human observation or knowledge—it doesn't constitute an explanation, it's just a fact. Epistemologically, the process of determining a defining characteristic will proceed by means of the question: which characteristic explains the others? Metaphysically, this means: which characteristic makes the others possible? Which is the cause?'
Which is the cause?
The important thing to realize here is that this is a strikingly Aristotelian thing to say, in contradistinction to her anti-Aristotelian comment pages before. For it is only in the Aristotelian context that one can speak to causes "from the inside" so to speak, or more technically, intrinsic causality.
For those unfamiliar with Aristotle's metaphysics, he held to four different kinds of causes: Formal, Efficient, Material, and Final. Science for the most part only believes in efficient causes, for example, billiard balls set in motion by some external force.
In the Aristotelian system, the example of a bust of Socrates is commonly used to explicate the four causes. The formal cause of the bust of Socrates is the form hidden inside the granite block it originated from, this is the form as viewed by the sculptor. The efficient cause is the action of the sculptor chiseling away at the block of granite. The material cause is the granite from which it is made. And the final cause is the perfect form the bust would take at the hands of a perfect sculptor living in a perfect world. It is not so much the bust itself but the idea which takes imperfect form in reality. That perfect, final form is the end which the sculptor strives toward in an imperfect world.
For Rand, a "defining characteristic" is that which makes the most other characteristics possible, intrinsically speaking. Rand's theory is just as intrinsic as Aristotle's theory of Formal causality, regardless of whether or not she would have viewed the example of the bust of Socrates as quaint or old-fashioned (which it is). Rand limited her definitions to actuals, it did not include potentials as the form of Socrates hidden within the granite block. Everything, for Aristotle, was in the process of becoming; for Rand, everything simply is.
That difference, however, is not important concerning the fact of her intrinsicism, it only detracts from the time element or the process-orientation found in Aristotle's metaphysics.