Greece introduced the additional test requirement for vaccinated travelers last December to curb the spread of corona infections. This solo effort by Athens was criticized by other EU countries at the time.
However, unvaccinated people still need a negative PCR test on entry, which must not be older than 72 hours, or a negative rapid antigen test that is no more than 24 hours old. This also applies to travelers whose vaccination certificate is no longer valid in their own country because they did not have the booster vaccination.