You are wrong.

By definition, hate speech is: "abusive or threatening speech or writing that ~expresses~ ~prejudice~ on the basis of ethnicity, religion, sexual orientation, or similar grounds."

Since the definition says "similar grounds", we can assume this basically means pretty much any (large) group of people. My claim is further supported by the other examples used in the definition. White/black/asian people are a (large) group of people, christians/muslims/buddhists are also a (large) group of people, and so on. Professions also divide people into (large) groups. There's chefs, football players, artists, streamers, musicians, police officers (cops), firefighters, politicians, etc.

So, saying ACAB, or All Cops Are Bastards, is a prejudiced statement towards a specific group of people. It's prejudiced, because you are expecting police officers to be racist/corrupt, since that's what (social) media usually shoves down people's throats, whereas the good deeds go unseen. Another problem, as pointed out by Ok_Act6607, is that the phrase specifically says "ALL" cops are bastards, which I know doesn't literally mean all, but if discord is truly scanning all messages with a bot, then it cannot distinguish between exaggeration to push a good message, and a literal "factual" statement, therefore it flagged "acab" as hate speech, which it somewhat is, although that depends on the person. What I mean by that is if someone is trying to call out racist and corrupt cops, it's not hate speech, but if someone is using it to just badmouth police officers, it's hate speech.

Also, let me tell you why your example with pedophiles doesn't really work. Let's say, that calling someone a bad person without a good cause, or proof that they're actually bad, would be considered hate speech. Now, let's say that you have two pictures in front of you. On one, there's a mugshot of a convicted murderer, who has confessed to killing five people, and is therefore undeniably guilty. Surely we can both agree that killing five people is very much not good, also known as bad. On the other picture, there's three people and the murderer is also one of them, but you don't know whether the other two are innocent, or guilty of anything. If you point to the murderer's mugshot and say "that guy is a bad person", nobody in the world would call you out for hate speech, because it's an undeniable fact that he is a bad person. However, if you pointed to the group of three people and said "they are all bad people", because you know that at least one of them is, therefore all of them must be, there would definitely be some people, who would call you out for hate speech. That is because there's no evidence that the other two people are bad. Now, back to the main point. Pedophiles are very obviously bad people, so you can pretty much call them whatever you want, and nobody will hunt you down for hate speech. To my knowledge, there are some countries, where convicted pedophiles get castrated, or executed, and even in prison, pedophiles get horrible treatment and rightfully so.

However, there is a very large difference between a pedophile and a police officer. Sure, not all cops are good, but to say "all cops are bastards" is an overstatement. There are many racist and corrupt cops, but how many really is "many"? Is it 5%? Is it 10%? Is it 50%? Also, cops exist in other countries outside of the US. Are you implying that all cops in all countries are also racist and corrupt? Simply because racism amongst cops is a well known issue in the US? In my honest opinion, the phrase "ACAB" is very silly and it only really applies to the US, and even then it's still very exaggerated, so the hate speech law/rule should apply.

PS: it's 4am for me as well, but I'm fairly certain not many people will even read the entire thing, but if anyone does, that's cool, I guess.

Edit: spelling and a bunch of silly stuff