It's the same definition from a talk by one of the PPO developers and also used elsewhere, i.e. it being first and foremost whether the output is inferred by ap

It's the same definition from a talk by one of the PPO developers and also used elsewhere, i.e. it being first and foremost whether the output is inferred by applying proper epistemology (justified correct belief) to its training data. It's a bit more nuanced than simply the negation of factualness (or 'correctness'). Yes, it means proper application of the term means you have to know what went into its training data (or current context), but you'd have to make those assumptions anyway to be able to put any credence at all to any of its outputs.