13 years ago... I assume this was mapped based on the rules used at that time. "alternative" sound like a different routing e.g. happening at some time of the day.

B20 ist ein Fehler, sorry. Muss mal recherchieren.

IIRC I didn't change the tagging of the bicycle paths, they have just been influenced by other changes (eg crossing ways). => please have a closer look to changeset. BTW: the chaneset was a year ago => very hard to remember such old changes!
BTW: oneway = yes is a clear tagging which says that it's a oneway. And: that's IMHO normal to a bicycle path if it's related to a road which is also oneway. If you drive it in the opposite direction you will AFAIK get a ticket by the police.