If you are talking about genes, then yes, you are correct.
They are talking about species, and then yes, you get into something of a category conundrum. One of the simplest, though not entirely true, definititions of species is that they don't interbreed. But since in this case we're talking about individuals living at different times, that's not even theoretically possible. So yes, we set a somewhat arbitrary date for one species to become another.
Still, I found the graph self-expanatory. Eventually what was a mutation or very unusual gene, can become common, first as something hidden or "recessive", but then rather suddenly, as it reaches a certain threshhold, general.