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."
Yes. DB doesn’t operate any sleeper cars. All sleepers operating in Germany are by international corporations (ÖBB, etc). But they aren’t covered by a DB BahnCard 100.
The trains he mentions are just regular ICEs that operate for some overnight trips.
I can imagine how this can be reasonably comfortable in an older ICE1 in first class, but i wouldn’t want to sleep on ICE4 seats for extended periods of time
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.