Remember that they’re not flying towards the Moon but to a point in space where they and the Moon will be closest together in a day or two, hence the Moon is now ‘off to their side’ and they can see a segment of it that is hidden to Earth observers … I think.

Also, the dark side of the Moon is often illuminated but we call it dark because it’s also hidden from earth due to the Earth and Moon being tidally locked (the same side of each always faces the other body).