My rational is that the process of cutting the thread reduces the wall thickness of the existing barrel by .075" which is not insignificant.
Because what is left of the old chamber has a taper from breach toward the muzzle end of the chamber of .004, when soldered in it is also effectivly wedged by a greater thickness of solder at the breach end than the front of the chamber given that the new barrel stub is paralell.
I see the threading then as more a convenient locator and means of centralisation than a method of fixture. Without threadind some portion it would be much more difficult to solder and centralise the barrels.
I cant see this shooting loose in normal use.

Have you any pictures of your project??

I bought a Powell back action ( 1910's ??) sidelock 12 bore the other day as a possible light double project if this one works out, but that will need lump as well as it is well off the face but it was cheep.
If I do make this one it may be all threaded as there will be more meat to start with and I have more control over thread sizes etc.

Regards and good luck