Rosenhouse’s belief in the creative power of evolutionary processes is based not on hard data but on his faith in the philosophy of scientific materialism.
It’s the evidence from Sagan’s fiction and other popular writing that is especially provocative.
Rational agents often detect the prior activity of other designing minds by the character of the effects they leave behind.
To summarize, the authors use a terrible design hypothesis, and make ever-escalating claims of certainty from two weak observations.
The evidence seems pretty light, at least compared to what we normally expect for a design inference.