I think that you have to have a dividing inlaid line between the brown damascus and blacked steel for practical purposes in finishing the different ways and that seems to introduce a weakness in the joint not found with laser or TIG welded joints. I am not clued up enough to justify that in a scientific way, just a feeling.
I like the look of the finished item, more because I know the challenges of achieving the end result rather than the aesthetic but it is clever and undoubtedly attractive.
As to Geo's point, I would say that since the internal of the sleeved tube is modern steel from end to end and the external damascus only forms the outer shell, I have no problem with the user using the tube as whatever level it has been proofed to.
To date, I have always blacked my sleeved barrels from end to end.
