For this explination to make sense, you must regard it within the context of the entire Pali Canon, meaningfully the Adhidhamma.
As these beings continually pass away into lower planes, whilst the world-system is forming during the evolution phase, the material of the solar-system is forming the sun and the planets. Remember that the brahmas do not percieve the light of the sun, they see by their own light which they radiate from their fine-material bodies. When the world finally becomes visible to them, and the darkness clears, and the sun appears, there is the world and the savory nutriment spread on the water. They partake of it and continue to become grosser. Whilst this is happening, the nutriment is developing into mold, fungi, plant-life, and subsequently the degenerating beings are partaking in these things and diversifying, and they develop sex. At last, the beings who've become animals become gross organisms at the bottom of the ecosystem, and consecutively this beings follow up into the humans. Thus the stable biological ecosystem is formed into the continued evolution phase.
Hopefully that was helpful? To me, this is clearly what the sutta is explaining.
Sometimes no Buddhas arise in the world. Sometimes they do. When it happens, it is for the welfare and happiness of men, out of compassion for all creatures. For a long, long time he has been working to become a Buddha. He met other Buddhas along the way. And after his long striving he attains his final life, yet not without showing everyone else how to get there.