The quote is taken out of context, but is nonetheless still confusing, even in context. Since the 81 must refer to radius2 not the surface area or the "new diameter you want to make" as he first makes it sound (I'd like to see the oven that 81" diameter pizza goes into!), that would lead one to believe that, since the root of 81 is 9, it must be an 18" pie, not 16" as stated.
Thanks for confirming my suspicions. Your help, too, is always appreciated!