Really it just implies humans and sayians share (relatively) recent common descent.
H. Sapiens and H. Neanderthalensis could interbreed, but possibly had some difficulties, starting to speciate at 500k-800k+ years of genetic seperation.
But Lions and tigers can still interbreed and female ligers are even fertile, and that's at like 3.8m years of seperation.
All of this means that Sayians probably share descent from at least Australopiths like the famous Lucy. They could share descent from something as recent as H. Erectus, or maybe they diverged from a sister lineage to Homo like Paranthropus.