"So when it's used to refer to a self-identified otherkin or therian individual, it's usually used to basically indicate that they're just a roleplayer and not genuine?" In answer to this question,
@Ambrose, that is mostly correct. Yet it is not limited to the roleplaying element, more often it can be said that the person in question is mostly attempting to be special or distinct in some fashion in order to gain social currency. No small number of people are looking to receive attention and being different, often radically so, is an expedient way to do that on certain internet communities and now bleeding over to reality. For them, it has reasonably nothing to do with actually
being that thing, if they so much as are, rather it is about getting
attention for it.
So in some ways they may or may not be roleplaying - pretending to be therian or otherkin - or wholly genuine, but that is less the focus. The actual thing of substance, them being that thing, is not what they are looking for or legitimately caring about. One could say a major issue with a person who is "all fluff" is that they are a "poser", to borrow some older language. Namely because while they are that thing, at least outwardly, whatever inner contents there should be are either nonexistent or severely lacking.