I agree that it would make sense to integrate a wiki into Lemmy. However that’s much more complicated than a standalone project, because it needs to integrate well with all the existing features. Plus it means more work for all the Lemmy developers, while Ibis could be handled by a completely separate dev team.
Also having Ibis tightly integrated with Lemmy would automatically limit the userbase to existing Lemmy users. The potential userbase for an independent project is much bigger.
That said it would be good if Ibis can optionally integrate with Lemmy, but I’m not sure yet how that would work. The most straightforward option would be to treat Ibis articles as normal Lemmy posts, but maybe that doesn’t make the most sense.