>>544954062
I believe he's referring to the proposal that the tyranids are not simply what is currently attacking the milky way, but that they are possibly numerous independent, galaxy-destroying hive fleets that float around the empty void between galaxies. Every galaxy they've successfully invaded completely stripped of water and any other building block of life.
They may be a threat to the universe itself, and it's impossible to tell how much of it they have already consumed. It may be the case that they've already consumed enough galaxies and spread far enough from their point of origin that the tyranids have achieved a critical mass that has doomed all biological life that is not tyranid.
I suppose a unified intelligence that might be destined to wipe out literally all life besides itself in the entire universe is high level cosmic horror. It's like a single celled organism that has grown into a completely psychopathic, malevolent god.