Have you done a sleep or diet study or are you just making assumptions based on your experience?
I think the Wikipedia page on night-owls explains in greater detail the exacts of what is essentially a genetic predisposition, the sources hold up too.
Anecdotally, I've tried many diets, I thought I had insomnia at one point and the doctors did not disagree, taking pills does not make it easier- you feel more disconnected and my usual grumpy demenour is more apathetic.
As with most things YMMV
1. it makes it hard to function in society; and
2. it can be treated or mediated somehow, so there's a point to having a name for it.
Basically, a "disorder" is a node connecting "symptoms" to "treatments."