Incorrect. Robotic probes piloted by AI and carrying the genetic material, or even the just the blueprints of that genetic material (some would say it's the same thing) or the originating civilization and robots advanced enough to build other robots to carry out various functions could be sent all over the galaxy at subliminal speeds and still have plenty of time to find life sustaining planets to colonize.
The UFOs people see would not be interstellar craft but rather more like shuttles built locally by the alien colonists. The raw materials to build could have been mined either by the alien robots, or by a species of the native fauna that had been genetically manipulated by the alien robots to serve as somewhat intelligent - but not too intelligent - worker bees.
