He is not. He's claiming there is a definition, and we don't have it, ergo, we cannot assume that X could fall on either side of the definition.

But the fallacy is in assuming that the concept has an objective definition. Assuredly, it does not. And so nothing can intrinsically fall on either side of something that does not exist.

Concepts do not have intrinsic legitimacy. Concepts are things we invent via language. Don't be lazy. State your assumptions for the context in question. Discuss around those assumptions. Don't assert that your framework for discussing the concept is "the Truth".