Believing this was man-made requires a second belief, as ES says, that they had a cultural reason to depict something despite the fact they couldn't see it properly. So it's a big speculation. On the other hand, Orions belt at Giza or Thornborough could never be seen by their builders.
I'm not sure what could be the mechanism for this being genuine and it mentions, anyway, that beet is prone to variable areas of growth. It's chalk substrata apparently - so is this a chalk horse? Why would this cause the beet to die back? If the horse area was very compacted, the soil above would be damp presumably so if anything the crop mark ought to be lush green?
How they'd do it is intriguing as well. Could you transpose a drawing onto the large scale, without being able to stand back to check it? Or maybe you'd build a viewing tower? Uffington's a puzzle as well - did the artist stay at the bottom of the hill with a megaphone?