Bob, I am not saying the image will be sharper when the lens is closed down. I am saying one will increase the tolerance for lack of flatness in the film plane. Depth of field and depth of focus stem from the same optical principle - the range over which the circle of confusion still appears as sharpness to our eye is increased at smaller apertures up to the point where diffraction begins to influence sharpness.
I used to print 4X5 in a glassless a carrier. Edge sharpness was an issue until the lens was stopped down.
Likewise, I can make slight corrections to perspective by tilting the easel. When doing so, however, it is necessary to stop the lens down to maintain sharpness across the easel.