I've read this logic / reasoning in various magazine articles and online forums. I think that's how an amateur handloader / wildcatter might look at it but not a commercial designer / manufacturer of ammunition.
A manufacturer doesn't think "Well, we better grab some .30-03 brass off the storage shelf so we can neck it down to .277" and make some .270 Win cases because a .30-06 case is too short." A manufacturer can draw or shape a cartridge case to whatever length and shape they desire. "Based on a .30-06 case" to me just means the same head and rim size and
approximately the same overall case length. It's just a common way to describe and communicate the basic dimensions of a cartridge based upon a widely known standard cartridge as a reference point (the .30-06).
Just my two cents... Cheers! Bob F.