That doesn't follow.
By examining the ruins of ancient civilizations, we can assess their technological progress at the time the site was abandoned. Perhaps it's a lower-bounded estimate, but if we find axled wheels, we know they'd made it at least that far. If we could examine the remains, we'd know things like yield estimates and technologies based on isotope findings, etc.
Except if you could examine the actual site
before it was abandoned, you'd have much better data and wouldn't have to speculate. In this case, all the other facilities are still there to be examined and give a
better idea of how far along they are compared to an abandoned testing site. Which, again, they had every right to destroy at any time if they so wished, meeting or no.
That leaves aside the question of exactly how much value there is examine the site where they actually blew up the bombs, versus the facilities they used to actually build them.