Give aliens more time to find us

OK. In my earlier blog, I found an article says that we are highly likely the only advance civilization in the galaxy, otherwise we had been conquered by aliens. A new arXiv article says Aliens need a lot more time to find us (original paper). The author made an assumption that the alien spaceships travel at 1/10 of the speed of light. Each ship is capable of launching up to eight probes. It will take 10,000,000,000 years (3/4 the age of the universe) to explore just 0.4% of the galaxy. So be patient. Let's look at the sky waiting for our little green friends with no hurry. However, I have to say that assumption underestimates the alien technology, without challenging our known physical principles (e.g., speed of light, time machine…). How about the spaceships travel at 20%, or 99.9% of light speed? What if each ship can carry 32 or ten thousand probes, each sends off radio signals or more nanobots?