The story of "Kryp" and "Tonn" may be a legend, used to explain this pre-history. The early version of the legend may have been similar to the Greek myths about the creation of humans. Over the centuries, the legend was embellished. The "Kryp" and "Tonn" version includes a addition from Krypton's "space age", since it refers to space-travel and astronauts. It is also possible that the story was not a legend at all, but was an original story by Du-Vor.
That's definetly the simplest explanation I can think of: It's a made-up story..