This is one way to intentionally leak your capabilities to the enemy. Maybe I am cynical, but sharing that spy satellites are 'weirdly good at finding underground structures' right now is providential indeed.
These were spy satellites in the 1960s and 1970s Modern commercial satellites have imagery at least this resolution (it's just not free to researchers). Any "enemy" who has not discovered this is not a threat.
The first 10 words of the article:
"Archaeologists have used declassified spy satellite imagery from the 1960s..."
Cynical indeed given your quoted phrase doesn't appear in the article. Even the word Underground doesn't AFAICS.
Cynical indeed, given that the first sentence in their comment is invalidated by the first sentence of the article. Kind of depressing how matter-of-fact people are about articles they clearly haven't bothered to even start reading.
What makes you think that these images from the 1960s and 1970s see anything "underground" or sub-surface?