These statements have been in each game jam's description since SCREAM CITY and we've seen no content deleted for being too "nasty". It's a vague term because it's hard to cover everything in a rule. If you approach a topic with tact and CW you shouldn't run into any issues unless you decide to make a Nazi game or a game that glorifies sexual assault. The best way to avoid your game being removed is to talk about your game concept in the community (Discord especially) and get opinions.
Stating that the jam itself should be a "positive and inclusive experience for everybody" is absolutely necessary to set the tone for conduct in the community, whether on Discord or here. As hankworx noted, the statement pertains to the jam but not necessarily to games made with it. For example, many games contain jumpscares, and the experience of being jumpscared may not be a positive one for everyone. But one should CW their submissions accordingly so others can exclude themselves as they see fit.
All in all, please don't worry about being restricted in what you can make. The only issues are if you don't CW (we would ask you to CW with appropriate terms, not remove) or when you tackle sensitive topics in an insensitive way (talk to people in the Discord about our game if you're worried about that). We don't want to remove anyone's game, these are just important contingencies to have in place.