It's not the BBC - it's the appalling Daily Mail that originated this.
Today Greece's infection rate is 14.1 per 100,000 over a seven day period. That is way below the threshold of 20. Portugal is 14.4, and it's been taken off the isolation list.
There's one statistical point that never gets mentioned - 100,000 of which population (the base population or the larger population when visitors arrive)?
The biggest threat to Greece now is the increasing chance of Turkey invading and annexing the Eastern Aegean islands. If or when that happens, the FO will assuredly argue against travel to Greece, even if Corfu is 800 kilometres from Rhodes. I imagine Kalamata and Preveza airports would be closed to civilian traffic. I was on Rhodes last time war seemed imminent, and it was bizarre sitting on the beach with jet fighters screaming past overhead.