I wonder what procedure he was following? The only place I have seen the ream process employed was after you replaced the cups to provide a smooth properly angled surface at the correct depth after the new raw cup has been pressed into place. But unless you detect damage, once the surface/angle is established I don’t see reaming them again. both these procedures are in the 34-2 manual under fuel system, and cylinder head repair…
I have seen some DIY options for the cup install and fitment process as this type injector install(inj into malleable pressed cup) is fairly common. The tapered cone gives a very generous sealing surface.
swapping an injector, you would of course want to insure any carbon leakage was removed from the sealing cone surface, but the new injector should push right down into a cleaned already properly formed cone, as the references I have seen indicate…
I suppose they could also be lapped, but getting air into the cylinder would not be easy... You could machine a tool that matches the nose shape/angle of an injector(or make one from an old injector). You would also need a guide for the top of the hole to hold it true. If that tool shaft was hollow, you could put in air from the top…