what stops lemmy.world from connecting to them and sharing the same content ad free?

That's true, I hadn't thought about that, but I wonder if he thinks that maybe Tumblr isn't popular enough to see a big loss in doing that, since at most it would just get more interactions from a community that wouldn't have joined a proprietary social network in the first place, so the missed ads would have always been missed, while in this way he gains more "free" engagement

About the porn, they could probably defederate from the largest NSFW instances at least