There are two main differences I can see.
First, superintelligence can create a better utopia. After astronomical amounts of time, dath ilan may have the technology necessary, but I suspect that they would lack the understanding of their own utility function that FAI would have. They are also not immune to politics, and will act suboptimally because of that.
Second, there's a not insignificant chance of dath ilan being wiped out by some kind of existential risk before they're advanced enough to prevent it.
I agree with the idea that the AI will help with existential risk.
First, superintelligence can create a better utopia.
What I'm asking is "What would this utopia have in particular that dath ilan wouldn't have?". The next question then becomes how much better would a society with those things be than a dath ilan-like society. I'm having trouble imagining what the answer to the first question is, so I can't even think about the second one.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.