If it wasn’t meant to be analogous to prejudice based on immutable characteristics why did they make it prejudice based on immutable characteristics? Sea lions in the universe the comic takes place in are clearly sapient people, capable of complex thought, communication in English, and the ability to navigate human society. The person in the comic isn’t being “metaphorically racist” or expressing something “analogous” to racism; they’re simply being racist.

As you say, what the sea lion is doing is harassment, it is hostile, it is aggressive, and the racist depicted in the comic deserves it. It is model behavior, we should make people dread the idea of expressing racist views in public.

If you substitute the racist opinion for a mundane one, of course the comic changes and the sea lion becomes unreasonable. You’ve completely changed what the sea lion was responding to. If you swap all the Nazis in Wolfenstein with peaceful dogs who aren’t doing anything, now it’s a game about being a cop. Now you’re being completely unreasonable by shooting them. But that doesn’t tell us anything about the validity of shooting the original Nazis. When we look at the original context of Wolfenstein, shooting the Nazis is completely reasonable because they’re Nazis and not dogs.

Furthermore, even if we do substitute the racism expressed in the comic for a more mundane opinion, like enjoying pineapple on pizza, it still seems inapplicable to internet arguments. A significant part of the reason the sea lion is unreasonable is its persistence. At no point do the other people in the comic engage with the sea lion but it won’t stop harassing them. But on Lemmy, this behavior is so universally punished that it almost never happens because it would obviously get you permabanned from your home instance no matter what it is. Most people I’ve seen accused of “sealioning” are simply replying, one comment at a time, to people who are actively engaging with them. You can’t be “sealioning” unless you are following someone who isn’t engaging with you somewhere else, meaning in this context a different thread, to demand further engagement in the conversation. Which, again, would get you banned so fast that I don’t think I’ve ever actually seen it happen on Lemmy.

On no level do most accusations of “sealioning” seem to hold up to scrutiny, in my opinion.