Interesting paper! thanks for posting that...
Ventus, to break it down, the Oswald factor is used , along with a myriad of other calcs, to estimate the wing span, and wing span efficiency in aircraft design.
I did a broad brush look at it, especially with some of the assumptions, coupled with the data provided and the corresponding source.
I did find some interesting issues:
1. the inviscid part. The assumption that the induced velocities from the wake or vortex drag are inviscid is not currently accepted by everyone any longer... I note how much of the foundation of the calculations is based on the inviscid parameter.
2. Corrections for span loading due to the fuselage ignore centerwing parameters, it simply uses fuselage diameter. I feel this is a significant overgeneralization of the component couple.
3. Winglets. It appears that they use the assumption that winglet height has the same effect as span increase, with a variable. This is oversimplified to the point of being useless. There are far too many variables associated with winglets and when they are efficient.
4. They reference specifically Boeing data, so it comes as no surprise that the paper shows the 737-800 having more drag reduction at cruise that the A320 Neo....