I think that stuff was not a reflection on forums or moderation in general but specific to the time that Slashdot was born in.
Rob Malda had a very strong pro free-speech stance and so the site developed this odd sub-culture early on that was very self-referential and filled with Slashdot specific memes. In fact Slashdot had memes before memes even became a widely used word. Natalie Portman+hot grits etc.
I think it attracted people who enjoyed the thrill of being able to "publish" anything at all. I am very skeptical those people actually said those sorts of things or even believed them in real life, because most of the racial troll posts were almost entirely template and extremely repetitive in meme-like ways, most of them were barely coherent: they weren't people genuinely trying to convince anyone of anything.
There was a second factor that I suspect was also 'encouraging' trolls at this time: Slashdot's combination of "We never delete a post for any reason", allowing anonymous comments and a very sophisticated moderation system turned it into a game for some people. There were people who just enjoyed trying to break the site by doing things that you don't see on any other forum. For instance there was a period where people kept coming up with creative abuses of Unicode to construct page widening posts, stuff like that. You had a lot of creative programmer types faced with something that was an utterly pure system, designed to stop abuse with no human-level decisions to beat, and no legal penalties for trying. So it turned into a form of lightweight hacking practice, both machine and social, without any actual legal risk.
These days people who enjoy finding exploits in computer systems for the sheer thrill of it have plenty of more profitable outlets, like the zero day hunt programs run by big companies. If someone were to try creating a new Slashdot and recapturing the glory days I think they'd be faced with a totally different set of problems.