Exterior lighting: Is a round waterproof lampholder ACTUALLY compatible with a rectangular housing?
The product description on Amazon is pulled directly from the manufacturer's data sheet (the link is an excerpt from a catalog, scroll down two pages for the light fixtures you bought), so I think you can trust it.  

Another assurance that you'll be OK: I see from that catalog that the base of the light fixture is 4 1/8" in diameter, which is the same as the outside dimension of the diagonal of a single gang box (at least the one I picked out of my big box of electrical goodies :).  When you open the packaging, you should see a rectangular pattern in the molding on the back where the electrical box will be mounted.