Putting any materia in a piece of gear increases its spiritbond rate by 20% (pentameld will double it, in other words)

I put dumpy materia in the 4th/5th slots all the time for this reason (low tier materia also has a slightly greater chance to be successfully melded). I still try to at least make it relevant materia to the role, but I'm guessing he just didn't have any low tier crafting materia on hand and didn't feel like buying any.