Quick update: you were right about going through transit and then changing terminals airside. And you actually saved my ... with my luggage as I indeed found it on the carousel in Beijing, despite being labeled all the way to Busan (never trust anyone in TXL...). Thanks a ton!
Cheers from Beijing!
PS: when I was transiting there actually was a sign for international transfers airside, but that exit was closed without any additional info. Maybe that's where the ominous airside terminal bus used to operate?