I work in a similar position at another job. Each place is different and I can only make an educated guess based off my experiences. That said, you have a defined target for harassment “Your dumb,” and “Fuck off,” is genuinely not targeting anyone without context.
That last part “without context” is important. As I said I can’t say for sure because I don’t work there, but guessing by the results you claim; the person deciding the violation is only getting the reported text and not any context conversation that goes along with it.
Another thing to consider is when only reading individual content without context, is that making assumptions based on bias is a bad practice. You should only base a decision off of company protocols. So if TikTok has a clause that says a target is needed in order for something to be harassment; “Fuck off,” is far to generalized, even if the decision maker believes it’s harassment.
Hope that helps.