Judging a process based on a bad quality example doesn't give an accurate representation of any process' capabilities.
A sleeving fit of barrels intended to be welded should be a close slip fit. That is; the fit should be withing a few thousandths or less. Ideally, it would be an interference fit. No flapping in any area would be possible. A good fit is important with any of the processes.
Typical Sn/Pb solder has about a 7000psi ultimate tensile strength at room temp. That drops dramatically as temperature rises. At 140F that drop in tensile strength is in the area of 30%. I don't have data above that temp other than the flow temp is 360F. More specific to the sleeving process is shear strength, since most of the surface area is on the o.d. and i.d. of the parts. The solder above, has about a 6000 psi shear strength. I don't have the decay on the shear strength vs. temp, but you can bet it tracks with the tensile. Remember, most of us will shoot a gun till the barrels are well above boiling at least on some occasions. Let's just be conservative and say you shoot it until it's hot enough to drop the shear strength by 40%. That leaves 3600 psi shear strength.
I'll swag the diameter of the sleeve joint at say... .900". Let's use 3" for the length of the sleeve joint. That gives us about 8 1/2 sq/in of surface. That gives us a strength of the joint of about 30,000 lbs. The problem with solders is "creep". At elevated temps, a sustained or repetitive force will cause creep. That's an incremental amount of strain (movement) do to time, load, and temperature.
On a weld, the ultimate strength is maybe 70,000 psi. The remaining weld after dressing down the outside is probably 1/16". The O.D. is about 1" at the 3" point on 12ga barrels. You can only weld about 85% around the barrels because they are close together. So, you end up with .16 sq/in of weld. Which comes out to about 11,000 lbs. But then there's the breach end that gets welded as well. That is probably worth about half of the other weld because it's in shear. So maybe 16500 lbs for the strength. Barrels just don't get hot enough to decay steel strength to any meaningful amount or at least not if you use the temperature we are comparing with the soldered example.
So, you're right that a soldered barrel is stronger if you consider that you only get the barrels hot enough to decay the solder strength by 40% and you have 100% solder joint for 3" of the tube.
The real question is: what are the forces pushing the barrel out of the breach assembly? I'm guessing they are pretty low. The success of both methods over the decades has validated this. It's just the friction of the payload going down the barrel and thru the forcing cone and choke from what I gather.
As you may have guessed, I sleeved a barrel set using TIG as the joint method. I was going for the asthetics of no visible joint.
