Yes, Apple can certainly stop you publishing an app in their App store, but Apple can't stop you telling people you intend to make an iPhone app (or accessory or phone case). And they can't stop you advertising that fact. I could state right now, right here, that I plan to make an iPhone app with no recourse.This is more like making an iPhone app, though - and if you want to do that, and have it carried on the Apple store, you so very much do need to play ball with Apple.
Roll20 can decide what happens in their store, but they can't dictate what people do outside it, including making claims of compatibility, as long as they don't do so in a way as to confuse customers into thinking Roll20 is making or endorses the product.
What they could say is "If you try to make a Roll20 module KS without our permission, we won't let you use the product on our platform". But that's very different to what they're actually saying.
So I'd be curious, too, whether this has actually happened.