My tagging here is not wrong.

1) According to:
https://wiki.openstreetmap.org/wiki/Key:access#access-dismount
it says:
Use bicycle=dismount when people are not permitted to cycle (e.g., through a graveyard) but are ALLOWED to DISMOUNT and BRING THE BIKE. See Bicycle#Bicycle restrictions. Note that bicycle=no is also commonly used where dismounting and PUSHING BICYCLE IS LEGAL.

2) According to DE:250
https://wiki.openstreetmap.org/wiki/DE:Tag:traffic_sign%3DDE:250
it says (the same thing !!!):
Verbot für Fahrzeuge aller Art
Anmerkungen: Gilt nicht für Handfahrzeuge, abweichend von § 28 Abs. 2 auch nicht für Tiere. Krafträder und FAHRRÄDER DÜRFEN GESCHOBEN WERDEN.

Therefore = BOTH
vehicle=no
AND
bicycle=dismount
IS CORRECT

... but I will alter it, so that everyone is happy