This is exactly the explanation one would give that doesn't understand the dynamics of the barrel pulled with great tension against the end of the arbor. Without the contact and tension, the end of the arbor could (and will) be pulled and torn off. The fact that static force is constantly pulling against the end of the arbor is what keeps the arbor from separation. If there is space between the end of the arbor and the bottom of the arbor hole (space for separation to happen) separation can and will happen.
For this reason, an Uberti 1860 Army can maintain it's build tolerances and handle a steady diet of 45acp +p ammo.
It's in fact, a BRILLIANT design that works perfectly if executed correctly.
So technically, since the wedge actually mates the barrel and the arbor together ( as a single unit), the barrel IS screwed into the frame ( since the arbor is screwed into the frame) and of course the cylinder is mounted between the two . . .
Mike