He's not saying that he only bought one spool of #10. My read on this is that he didn't want a green spool in two sizes, a black spool in two sizes, and a white spool in two sizes so he went with all #10 to cut the spool count in half.
Besides, he could have bought one spool of #10 white and still wired everything to code with a little extra effort. And if he didn't want to strip the white insulation off to make bare wire for the ground he could have used EMT for his conduit and skipped the ground wire altogether!