I just used this benefit at the Lufthansa lounge in Hamburg (HAM), when flying on a basic fare with Eurowings. The agent was unaware and initially tried to turn me away, but then allowed me to scan first my United card (*A gold), followed by the boarding pass. The machine then flashed green. The agent was surprised, but of course then let me in.
It’s strange that I can no longer find this exception listed on Lufthansa’s website—did their detailed lounge access policy disappear?