Orientation of halogenation
The statement is:


  This[collision frequency] must be the same for the two reactions, since both involve collisions of the same particles: a propane molecule and a chlorine atom.


It is clear that n-propyl radical and Isopropyl radical have same carbon structure. In the next step the hydrocarbon radical reacts with a chlorine ($\ce{Cl2}$) and since chlorine is symmetrical the collision frequency cannot depend upon it. It should be easy to judge from the following structures, the very minor difference:



Another reason is: Since the difficult step is the abstraction of a hydrogen and the determination of rates should be done while comparing primary, secondary or tertiary radicals, which have different stability.