I suspect ZDF would have argued as both them and SRF were both members of Eurovision/EBU and rights holders to the tournament, they could 'use' the coverage until the back-ups were up and running either locally at the ground or the IBC as the SRF feed was the only stable one available at the time. If something happened - overdub afterwards, and credit 'Das Footage von SRF'.
It does happen occasionally. I know of one football commentary which was being carried 'off air' for goal archiving purposes, where the BBC LR commentators ISDN codex box - or the line connector failed. He asked if they could dial up the rivals ILR ISDN and 'listen in' (they were carrying the game live) back at base whilst the meltdown was fixed - so they didn't miss anything.
All very cordial, and the BBC were back up and running after about 2 minutes - and they had no need to use any audio from the commercial radio commentary.
Not my station, I hasten to add - that would never have been sanctioned.
I heard the 'discussions' about what had happened on our effects mic after the game, from the BBC reporter concerned, in a conversation to someone else across the tables in the press box.
My user name might look like Hatton Cross, but it's pronounced Throatwobbler Mangrove.