I'm very confident that iOS 13.0 GM is what is being installed on millions of 2019 phones that are currently in build, or finished and boxed in secure vaults in CN.
Apple requires a "manufacturing build" for these phones whether the OS is 100% baked or not when production revenue phones are built. It becomes a logistical nightmare to open boxes or to re-flash after manufacturing.
I've used the last release of iOS 13.0b on my one of my iPad's and agree it's far from GMc quality.
I believe that 13.1 will go out the door a several days before the new phones land at doorsteps and at stores. The new phones will come out of the box with 13.0 and these users will update to 13.1.
This is how things work.
Don't ask how I know

.. and yes, I know.