Yeah I wouldn't have guessed that at all. My only familiarity with the idea of a dunk tank is white people doing it to each other voluntarily as some kind of fair/fundraiser thing. I always thought the idea of it seemed kind of sadistic (not that I knew the word sadistic back then). But I'm taking it to imagine hexbear's dunk tank is similar to the shitreactionaryssay that lemmygrad has, where it's posting takes of people who aren't even present, and in that sort of context, it seems more like throwing fruit at a picture of someone than anything akin to putting down someone who is trapped there and has to take it. There's also "dunking on" in the basketball meaning, which is sometimes used to refer to putting someone down on social media, like ratio'ing a person on twitter. That one is more directly humiliating, but doesn't use the term "dunk tank" and so unless the basketball meaning also has racist origins I'm not aware, we're looking at phrasing that can quickly overlap and could be confusing to people to take as inherently bad or racist to use.
If it was just a name change and it was an easy thing to change the name of without messing up links and existing activity and so on, I'd say, whatever, make it a name they don't feel icky about. But the internet often doesn't function that way. Screwing over logistics of user activity for the sake of feeling better about a name most people won't even know the real origins of seems like an odd decision, to say the least.