I don't think this is mathematically correct. You are sampling at double the resolution with a larger window.
The increase in resolution is from using 4, or 6, shots at 39 MP each to upscale to a single larger file size. Although it is not 100% similar to upscaling from a single capture, it is still relies on multiple native resolution of 39 MP shots to get the job done. Therefore the resolving power of the lens does not need to be any better for a higher resolution multi-shot image then it needs to be for the native 39 MP single shot image.
On top of this, there is no interpolation being used here. I did not want to explain this before, but will now. Since each pixel only sees either red, green or blue in a single capture, the computer uses the neighboring pixels of opposite color to the one in question to interpolate, or guess, the missing two values for that pixel, for every single pixel. This presents a number of problems, one in particular is for pixels at a sharp edge in images where there are two drastically different shades on either side.
Here, since each pixel at or near the edge is having its missed color values based upon pixels on both sides of this border, the computer does not exactly know how to proceed. So, depending on how drastic the shades are, the interpolation will create graduated values for these missing colors and will, in effect, create a slighting blurred edge.
With a multi-shot capture, the sensor moves one pixel per shot, so the computer knows the exact value for all three colors for each pixel. There is no guess work involved, thus the problem described above does not occur (and a few others too).
Now, typically, most photographers will not even notice this, even at 100%, since most have not seen a single capture of the same image compared to a multi-shot capture. But, if you ever do, it is very noticeable and makes the image look sharper at 100%.
As a matter of fact, currency is designed around the blurring created from interpolation to deter counterfeiters. Fine details in money often do not show up in the image, or not sharp enough to print from. With a multi-shot capture, all the detail is present.
So, because of this, even an okay lens on a multi-shot capture will make a sharper looking image, due to no interpolation at work, then a great lens on a single shot capture.
Therefore, we can not make assessments about the quality of lenses based on multi-shot images for single shot captures that are approaching the same resolution of that multi-shot capture.