Probably strictly true, but not practically speaking. Why bother to load powder that's not going to be burned anyway? Load less but faster burning powder. Handgun ammunition is designed to be fired from handguns, rifle ammunition in rifles. Both are designed to be used in the most typical barrel length. Reloaders can tune their loads to a particular length, and even to a particular gun.
(I have heard a few shots that went 'poof' instead of 'bang'. It's scary when the shooter pulls the trigger a second time - hoping the bullet at least went out the other end.)
If you have the time, experience, love of experimentation, and are willing to go for it - you are absolutely correct. Most of us do, or in my case - did, enough research to get a favorite or two for their most frequently used weapons - or for the deer gun they use once a year. Everything after that is a labor of love or maybe a worrisome obsession.
(Even though it never occurred to me to load specifically for a Commander or Officers Model, when I count it up that way I still get a minimum of nine plus various for 12 gage).
Also, the original (and still) definition of a sub-machinegun is that it uses pistol caliber ammo.
Carbines were traditionally been built around pistol caliber to allow for longer ranges & better punch without carrying specialized ammo.
Carbines also come by simply shortening the barrel of a rifle to make it make it less cumbersome while using the same (longer burn - big flash) ammunition.
(But) M1 carbine is an orphan sub-rifle round for which some pistols were subsequently chambered - reversing the norm and never even suggesting a rifle.
More to the point, the average shooter goes out and buys his or her ammunition from commercial offerings or from the local range. "Results may vary" and barrel length is a totally independent variable. In the latter case, range loads, they probably loaded what was handy so dirty is the norm, consistency is in your dreams, and check the casing before loading each round.