You are fundamentally incorrect, it is intended for fictional scenarios which could not happen in real life. It is the kink equivalent of The Bechdel Test. It is proposing the absolute minimum of what is needed to ensure that the kink is #nonproblematic: Is it old enough to grant informed consent, and does it have the capability to communicate that informed consent clearly to its partners? If it fits both of these criteria, it passes the JHT and is ethically fine, though thinking it's gross or not liking it is still totally fine--it isn't saying "It's ethical so therefore you're a bad person if you dislike it. It is saying It's ethical and so therefore you should not base your disliking it in the framework of morality and ethics.
Basing your disgust of sexual fetishes and kinks in morality is fundamentally a Puritan concept: It creates the idea of wrongsex, and it justifies a framework of punishment due to perceived moral high ground. It says "You are a better person than someone who wants to fuck Scooby Doo for whatever reason, because that person is inherently morally disgusting for wanting that".
Basing your disgust of sexual fetishes and kinks in a personal dislike is fundamentally the correct ethical response: "I am not a better person for not wanting to fuck Scooby Doo for whatever reason, and someone else would not be a bad person if they wanted to fuck Scooby Doo for whatever reason".
You say that it isn't for "sapient animals", and I fully agree! Regular degular nonhuman animals cannot give an informed consent to us, be it through body language or power dynamics baked into the species difference! There's also the fundamental question of "Is this body language clear because of informed consent or because of biological nature", which is always "No, if it cannot not want to consent, it is your responsibility to ensure that its boundaries are not crossed while it holds impaired judgement".
But Scooby Doo isn't a sapient animal. Scooby Doo is a fully grown Great Dane, yes, but he speaks fluent-if-accented conversational English. He's also clearly intelligent enough to use that ability to solve criminal investigations, which gives him the ability to offer that consent in a clearly communicated way, ie human language.
That's it. I don't wanna fuck Scooby Doo. I think the concept itself is innately hilarious, because of all the things you think are sexy, you landed on the cartoon mystery-solver dog. And I chose him for this example of what passing the Harkness Test can look like explicitly because he was the most incendiary take I could think of. He is an emblem of "Just because you hate it doesn't make it immoral" in this context. It forces you to confront the possibility that you hate certain kinks because you think you have a moral high ground, and why that morality-based justification is completely false and ethically unsound. But it lets you keep your own personal boundaries--something passing the Harkness test doesn't require your approval or even your neutrality. You can totally loathe it. But you have to loathe it the same way you loathe a type of food: There is no good or bad involved. It's just not for you.
