This is exactly what I told them.
I told them that I would, since I now know about it, adhere to this rule, but that I did not know about it when I made the second claim (not sure if it was in the T&C back then) and that it was up to them to check properly.
Since they did not do that, since it was long long ago and since I have a written confirmation of a free night, I accept the night to be free.
Let's see what they respond. Looks like they escalated it to a supervisor because the first reply came within 30 minutes, the second one is now at 15 hours.