My German is worse than non-existent so I have no idea what the text in the graphics is saying. However, the graphics appear to suggest that the writer does not acknowledge downwash ? Looking at the cited url, I gather that the researcher is a bird flight man. I have read a bit in that area and, interestingly, the natural world flight researchers appear to have a generally better understanding of matters relating to aerodynamics than many of our aeroplane colleagues.
As to why the pilot mantra reflect thinking from a hundred years or so, ago ? Who knows ....
The following youtube video might be of interest to those who haven't seen it or something similar ..
In addition to Arvel Gentry's papers which were referred to earlier in this thread, a related (to the above video link) AeroSoc paper is useful ... no, make that essential, parallel reading (Ackroyd was a very well regarded aeronautical academic) ...
https://www.aerosociety.com/media/48...historical.pdf