Speed bump
You make the assumption that the plug would be fitted from the outside, there is no technical reason with a well executed scarf joint that this should be any weaker than the original structure.
There seems to be the assumption on these pages that a scarf joint is weaker than the original structure....... It is not weaker or stronger it is exactly the same.
My preferred option would have been to fit the new section as a plug from the inside as this would be less disruptive to the outside of the aircraft.
All this assumes than there is enough room to get the required surface area to perform a scarf joint.