. . . and the reality behind that illusion is, well, the initial illusion.
Richard Dawkins famously declared that the appearance of design in living things is overwhelming. Theistic evolutionists do not disagree. But, like arch-atheist Dawkins, they assert that the appearance of design is an illusion and Darwinian evolution is the reality behind the illusion.
But TEs don’t agree with Dawkins about everything; they are, after all, “theistic.” And to give themselves a fig leaf for the whole “theistic” part, they assert that at a deeper, empirically undetectable ontological level, God directs the process toward an end.
In summary, the TE position is that the reality behind the illusion is itself an illusion, and the ultimate reality behind that illusion is the initial illusion, i.e. design.
The twirling sound you hear is William of Ockham spinning in his grave.
See also here.