Looking at that weld, it is a machine weld, done with parts in a jig. The slip clutch tube slides over the toothed shaft and both are locked in place by a jig. An automated mig head then welds the joint as the jig rotates.
In this particular weld, the welder head location was slightly too far over the tube. The weld melted the tube without heating the shaft enough to bond with the weld. The error was probably 1/8 inch or less.
I wonder how many pieces they produced before their inspectors detected the problem? Apparently enough so the defective parts had gone to the factory floor and into production tillers. And they did not want to pull apart completed units to inspect the shaft and weld. Just sent them out to the customers and let them find the problem. Probably only 25% of customers have rocky soil, so 75% will never complain during the warranty claim period. That is a pretty good ratio in favor of King Kutter.
A mis set slip clutch should never be able to cause a failure like that. If that joint could fail with a slip clutch set too stiff, it should be redesigned. However, it did act as a type of shear pin for you. What would have broken if the weld had not failed? Make sure your slip clutch works.
A neighbor was tilling and jammed a big rock in the housing, locking the tiller solid. He had a rusted solid slip clutch. The parts which broke were the PTO drive gears in the tractor transmission. $4000. to replace them because the case had to be disassembled to replace the gears.
If Kuhn were sent a picture of a failure like that on any of their tillers, they would send a free replacement no matter how old the tiller.