Yes, I can see that, because then the completed boiler/PV must be fully hydrostatically tested in the field and many of the pressure retaining welds may or may not even be accessible after installation.
However, I believe the Code must allow for this option, as many PARTs (with pressure retaining welds) can't be hydro tested till they are installed. A head manufacturer might have to make a large head by 2 pc construction and Code stamp the head as a PART, but he cannot hydro test just a single head in the shop, so the Code must allow for PARTs to be made without necessarily performing the final hydro test prior to shipment. Since this option is possible, I think you are exactly correct when stating the Owner/User or even R stamp holder purchasing the PART needs to specify the Code hydro test be performed in the shop.