This paper posits a path to colonizing/probing the universe with lightweight (30g) or heavyweight (500t) probes based on 3 propulsion methods (fission, fusion, antimatter) by mining a planet like Mercury to make a Dyson swarm to power the effort, and launching probes at 3 speeds (0.5, 0.8c, 0.99)c across the universe with sufficient redundancy to survive intergalactic/cluster dust collisions.
Once arrived at target galaxies/clusters, they build a new factory system to launch another generation of probes to visit locally.
The fact that we have not seen evidence of such an operation does not mean it has not happened - the paper posits hiding such an effort would be trivial and additionally quashing similar efforts by other less careful aliens might be beneficial to a first-mover to avoid paperclip-maximism of the universe. Thus there may be a hidden universal police in place. Or simply too much intergalactic dust to colonize a good chunk before Hubble flow makes targets inaccessible.
Obviously assumptions about technical capability have a wide range of error - can we send smart enough AI with minimal tool sets (in 30g? in 500t?) to start a new Dyson swarm and probe factory? I'm more concerned with the light-speed travel, dust density, Hubble flow, material longevity and other more physical assumptions made.
There are multiple assumptions made, just as in the Drake equation itself, some of which likely have even wider (magnitudes) range than used in the paper.