That's my understanding, too. Just speculation on his part.
There seems to me a scale here, where we go from science to speculation:
1) Micro evolution: something fairly well established by science.
2) Theory of macro evolution--species can change from one to another: the prevailing theory which actively ridicules competitors...but not really a hard science. The empirical evidence lends itself to hand waving arguments both for and against. One can attempt to apply the scientific method to test this theory, but there are too many confounding factors to make such investigations definitive (as there are in psychology for instance).
3) Assertion of a single common ancestor of all species: realm of speculation. Not really testable.
4) Presumption that that single ancestor must have sprang from inorganic matter by some kind of chance (either here or on some other planet): Wholly speculative, neither testable with science, nor something which can be derived from reason.
5)Problem of where matter/energy/stuff itself came from (for which life could later spring). A compelling philosophical case for a transcendent super nature. But not something which is testable by science except on occasion and indirectly. For instance the Big Bang theory which suggests that the universe has not simply always existed--a notion which if accepted forces a reasonable man to reject naturalism.
So on the one end we have philosophy and the other end we have the scientific method as the best ways to approach the questions. In the middle is no-mans land.