This has nothing to do with entry into the country but entry into restaurants. My understanding is that the State of Hesse, where Frankfurt is located, demands a vaccination certificate, a proof of previous infection, or a test which is maximum 24 hours old to enter restaurants. The lounge dragon told me that it's 24 hours for all tests. McDonald's had a sign saying that you need a 24 hour test to eat in. Take-away was still possible. PCR tests would make it even more difficult, as it takes longerr to get results.