Coincidentally, I was reading the quantum non-realism article when writing my recent understanding your understanding article, and that's where it's from -- though he mentions it actually happened in a previous discussion and linked to it.

The context in the LW version is:

My attitude toward questions of existence and meaning was nicely illustrated in a discussion of the current state of evidence for whether the universe is spatially finite or spatially infinite, in which James D. Miller chided Robin Hanson:

"Robin, you are suffering from overconfidence bias in assuming that the universe exists. Surely there is some chance that the universe is of size zero."

To which I replied:

"James, if the universe doesn't exist, it would still be nice to know whether it's an infinite or a finite universe that doesn't exist."

Ha! You think pulling that old "universe doesn't exist" trick will stop me? It won't even slow me down!

It's not that I'm ruling out the possibility that the universe doesn't exist. It's just that, even if nothing exists, I still want to understand the nothing as best I can.

(I was actually inspired by that to say something similar in response to an anti-reductionist's sophistry on another site, but that discussion's gone now.)