They say we know more about the Moon than about the deep sea

We know more about the Moon than the deep sea.
This idea has been repeated for decades by scientists and science communicators, including Sir David Attenborough in the 2001 documentary series The Blue Planet. More recently, in Blue Planet II (2017) and other sources, the Moon is replaced with Mars.
As deep-sea scientists, we investigated this supposed “fact” and found it has no scientific basis. It is not true in any quantifiable way.
So where does this curious idea come from?
They say we know more about the Moon than about the deep sea. They’re wrong.