I am amazed by your support to your German colleagues! What would you have told whether the incident would have been with Wizzair, Ryanair or similar operators?
This being said, when referring to ICAO, I agree there is no definition of a "closed runway", but there is a definition of a "runway":
A defined rectangular area on a land aerodrome prepared for the landing and take-off of aircraft.
The less we can say is that that rectangular area was not exactly "prepared" for landing. But you will likely challenge me on the fact there is no definition of "prepared".
I think that we can consider that what you call a runway was not an operative runway; effectively, when a paved runway is to be operative, the ICAO requirement (not recommandation) is that "A runway designation marking shall be provided at the thresholds
". The definition of the required marking is given in the paragraph 5.2.2.4 of the Annex 14.
I fully agree with you that anybody can make mistake - and in this event, I do not blame the crew - but your support to this mistake is beyond the reasonable, simply because you try to use unreasonable arguments.