The 2018 reference in "Space Seed" does dangle there, since it seems obvious that any early-21st century Earth vessel predating Cochrane's Phoenix flight would have to be sublight in nature.
My take is that prior to 2018 sleeper ships were required for travel to any interplanetary travel within the Solar System (Similar to what we see in 2001: A Space Odyssey). In 2018 there was a propulsion breakthrough (impulse, perhaps) that--while still sub-light in nature--made sleeper ships for interplanetary travel. They would still be needed for any attempted interstellar travel until the development of the Warp Engine...
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy