If you say so.
See if you can find where the law says so.
A manufacturer can manufacture barrels any length the manufacturer chooses.
A manufacturer - or anyone else - can cut a rifle or shotgun barrel to any length he chooses.
A cut barrel is just a cut barrel and has no particular legal status.
If a barrel cut to below the legal minimum is installed in a firearm, the firearm becomes prohibited.
There is no exemption in the law for a manufacturer to install shorten finished barrels which have been cut shorter than the legal minimum in a firearm without it becoming a prohibited firearm - unless the manufacturer is licenced to manufacture prohibited firearms.