That just means that Mew doesn't reproduce through normal means. IIRC, there were many "Mew" at one point in time. The odds that a notable populous has no reproduction capabilities at all would be questionable, but entirely possible given Mew's simultaneous support of both creationist and evolutionist origins of the world.
It would also work into the "unstable Mew become ditto" theories as well. --- Follow me on Twitter: twitter.com/Hasire Watch for videos on my YouTube: youtube.com/hasire [under "construction"]