Probably a good and accurate observation. I used the facility Paris to Frankfurt (south station) with my LH boarding card as a ticket during a strike which was either French ATC or LH pilots or cabin staff (can't remember) about eighteen months ago. No problem with it being accepted at the time, but perhaps a degree of mitigating circumstances. I sat the whole way in the restaurant car (IIRC the rest of the train was sleepers only) with a very pleasant fellow who went on to Berlin with the same kind of "replacement travel documentation" that I had. Maybe I this was a situation where there were deviations to the norm.

That said as you accurately point out you can get to Berlin quicker by changing in Mannheim or Cologne if using rail travel, although flying is likely to be quicker and cheaper