Doesn't focussing distance influence the performance of a lens?
Yes, camera to subject distance will affect performance.
Leica's official stance is that any lens testing should be done at 100x the focal length of the lens, although on occasions they've relented and said 50x is acceptable. And if Leica say 50x is acceptable then I guess a bit less than 50x is probably acceptable too!
When I first saw this test I thought a 15.5" target filling the frame was a bit on the small side. But if it was a problem then we would have seen the macro lenses take an advantage. That didn't happen so, no harm no foul.
I'd have thought the bigger advantage of shooting this subject from a longer distance would have been to better illustrate any differences between the cameras and lenses used. But maybe the studio dimensions didn't allow for this.