Comment by bradyat

From his blog: "I spent many nights trying to figure out the best way to sleep in first class. In the end, sleeping on an air mattress under the seats proved to be a good idea."

https://leben-im-zug.de/meiner-erster-monat-mit-der-bahncard...

So I guess he really is just sleeping in the regular compartment rather than a sleeper berth.