They could just as well say that whatever created horses created unicorns too.
The multiverse advocates’ project is not to undermine the evidence base as such. There just isn’t any evidence for a multiverse. Their project is rather to undermine the idea that evidence, as used in normal science, should matter in cosmology. String theory, we are told, is useful even if unconfirmed (Quanta). Supersymmetry is beautiful, lacking only supporting evidence (The Economist). The multiverse is a done deal anyway (ScienceBlogs).
It bears repeating: Advocates do not merely propose that we accept faulty evidence. They want us to abandon evidence as a key criterion for acceptance of their theory. More.