You drive into a brightly lit tunnel which is indicated like this. What do you do?

This sign indicates the entrance to a tunnel. In Germany, when you drive into a tunnel, you are required to switch on your dipped headlights, even if the tunnel is brightly lit and even during the daytime. This increases your visibility to other drivers and improves overall safety inside the tunnel. Driving without lights or only with daytime running lights is not sufficient or allowed in tunnels. So, your answer is correct: always use dipped headlights in tunnels, regardless of the lighting conditions.