A theory is just someone's idea to explain something until it is proven or disproven. Unless we work for Kia Engineering or Quality Assurance departments we don't know what is in Kia's design specs that might allow for a variance of up to +/- 5% between the front and the back shaft joints at the time of installation. That might change the theory of a design problem to just a poor design choice.
It could be a poor design choice to use U joints instead of CV joints in the middle and ends to save money and offer a lower price point. Kia isn't trying to build the ultimate luxury off road vehicle but rather an affordable but profit making mass produced people hauler with an AWD option. Another possibility is that it is less than optimal passable installation standards that vary between AWDs as they roll off the assembly line that could be made worse by high demand and faster production speeds.
I'm not suggesting the one independent report that was done is not valid, but at this point with just one we can't accept it as anything more than one specialist's opinion examining one
VIN stating that they feel it could have been made better to mitigate vibration more. The report appears to simply state that there is a difference that would contribute to the vibration and that a CV joint instead of a U joint might mitigate the noticeable vibration.
If it is a widespread issue, then it's very possible that a CV joint and shaft swap could be the answer for a better vehicle. But remember U joints are an acceptable implementation for AWD driveshaft coupling and there is only a $2,000 difference in MSRP from FWD to AWD, and just $1,930 difference in invoice on a 2021 LX FWD vs AWD according to kbb.com. That would include things like: additional assembly time, all the electronics, the additional shafts, joints, the rear differential, fluid filling and testing, upgraded tires, etc.. So they had to cut costs somewhere to still profit on an AWD over a FWD. Even if they are selling AWD parts at cost (or a lower profit % from FWD) we have to assume you are getting no more than $2,000 in parts. If you look at a Ford F-150, the most common truck sold in America, I think the MSRP price difference between 2WD and 4WD is typically about $3,500. Granted the profit margin is much higher in a large truck, they might still only be making about 8-10% on invoice for that one feature. Just from looking at the fundamental design, U-joints are probably cheaper to make than CV-joints because there is no rubber boot or fluid. The CV-joint appears to be a ball in joint with a rubber boot around it. While U-joints don't risk getting punctured but they are less flexible because they function with less movement.
The big variable and unknown is that AWD vehicles tend to have some level of added vibration over FWD vehicles. We still haven't established a baseline for what is generally acceptable vs. what is unreasonable for everyone. For example if one person is moving into an AWD Telluride from a 4x4 or something like a Jeep Wrangler, I might think the same vibration annoying to one person not use to it is a dream to another use to more. If you didn't travel at speeds of 65mph or more regularly because of local speed limits and driving habits, one might hardly notice the vibration with or without the AWD traction enabled.
I would think the next step is to better identify and compare what the actual issue is so that the people who probably should look into expert diagnosis can learn how to initially identify the problem and diagnose it themselves. Once you can establish a common way to identify the issue, the next step could be narrowing it down. Is this issue happening on a specific trim or trims with AWD? LX, S, EX, or SX? If only in some trims but not all, is it tied to some other package or option that affects the weight of the vehicle that could impact the angle the shaft sits?