He said that particular way of buttering toast was a rational way, not the rational way. There are other ways of buttering toast which may be rational, such as dipping it in liquefied butter or hiring a chef to do it for you.

Or perhaps you enjoy inventing complicated gadgets, in which case you might build an elaborate Rube Goldberg device to do it for you.

That's the beauty of instrumental rationalism... we aren't constrained by the silly, petty notion that there's only ONE "correct" way to do something. ;-)