How did they locate these particular images? I would love to read something on that, or similar projects looking across satellite imagery for specific patterns.
Whatever approach you take, you'll probably be selecting the final set by hand, so it's just about building the candidate set in an efficient manner. Low absolute accuracy isn't really an issue as long as you end up with a managable set to review.
As you work through your day as a satellite image person (I don’t know the job title, sorry) and see a shape that looks like a letter screenshot and save it. You’ll have an alphabet in a very short amount of time.
Given the repeats of letters I’m seeing I think that’s all they’ve done here.