There is an earlier section in Fulgrim that refers to it as Ulthwe. You could make the argument that the 9-14 km ship was another ship but I do not think that this is likely.
I very much doubt that the Eldar population would grow the way some people have suggested. It seems logical that Craftworlds would grow as refugee populations are integrated into it which is probably the main source of a craftworlds population growth. Eldar birth rates are also supposedly very low due to their very long lifespan.