Well, it does, sort of. I recently ordered a 16 x 20 print of one of my daughter's bridal pictures. I cropped the image in Lightroom to the 4 x 5/8 x 10 ratio, and then exported a JPEG changing the size to the long side being 6000 pixels (6000/300 PPI = 20 inches). Then I sent that enlarged JPEG to the lab. The photo that was returned was exactly what I expected.
I guess I have just been using Lightroom for so long that if I can't do it one way, or at least don't know how to, I can always find another way to get the job done.