We tend to discuss (understandably) how we don't see the far side of the Moon, discuss the difficulty of the "radio shadow", and are delighted to occasionally see pictures thereof.
Consider the reverse: someone (yes, hypothetically) living their life on the far side of the Moon would be completely unaware their quiet home is just 1.3 light-seconds away from a brilliantly reflective orb teeming with life.
Well, all Moonies have to do to see the Earth is to travel to the opposite side of it, whereas to do the opposite from Earth requires launching a vehicle out of the planet's atmosphere, across the orbit of the Moon, and back to Earth again.
Granted, it wasn't until several hundred years ago that Earthlings began to circumnavigate the globe, but the technology to travel long (cross-continental) distances have existed since ancient times.
The furthest they could be is a point exactly center of the far side of the moon. If they travel along a great circle, they'd only have to travel a distance equal to one quarter to the moon's circumference to be able to see the Earth.
If they live not at the center of the far side - and if they choose the wrong direction (because how should they know?) - they can travel more than a quarter of the moon great circle.
Ah, good point. So really the at most is `0.5*circumference - epsilon` (starting just on the far side). In the worst case, if they manage to stay on a great circle.
From the article:
"Owing to various view angles from different parts of the Earth, the most ambitious jet-setting astronomer could have seen a maximum of 59 percent of the surface of our planetary companion. "
If we take into account that fact that Earth is not a point, then epsilon is not so tiny after all, it's around 0.045 times circunference.
This would make the final result 0.455 times circunference.
But they'd have to know which direction is the correct direction, and they wouldn't know which direction is correct until they knew where the Earth was.
At one point there was a theory that there was another Earth, in the same orbit as ours, but diametrically opposite so that it was always hidden by the Sun.
There may actually be one. I'm not aware of anyone who has wasted expensive spacecraft time, energy and bandwidth to point an instrument in that direction to look.
If there was anything of any size there, we would be able to measure its effect on the rest of the planetary orbits.
Also, it would not be in a stable orbit, and would eventually either get ejected from the solar system, move into a different, stable orbit, or intersect with us (as Theia presumably did).
Consider the reverse: someone (yes, hypothetically) living their life on the far side of the Moon would be completely unaware their quiet home is just 1.3 light-seconds away from a brilliantly reflective orb teeming with life.