You are right, just saying "he does not make new assumptions" does not explain the author's act quite well enough.
On further deliberation, I have come to the following conclusion: there is a difference between assuming "only physical objects exist, and they are finite" and making an argument of that. The author is trying to say he is not making an argument for the validity of the axiom but implicitly says he just assumes it, to guide the focus of the reader to the main argument he is making.
I acknowledge that you definitely do not need to make that assumption, but as I said, you have to start somewhere. I'd say the metaphysical assumption captures a substantial subset of all physics, hence his ideas are worthy of consideration.