Is the bold part of the following sentence grammatically correct?

That sentence doesn't work as it stands, because there is nothing that "revealed by the village elders" could reasonably be taken to describe.

The following is better:

It was naturally assumed the temple was burned down as an anti-revolution symbol in those dark days, but, as was revealed by the village elders, it was burned down under much more mysterious circumstances.

That version implies that the elders revealed the latter fact. But it doesn't imply more specifically that the elders revealed it specifically to the narrator -- for all we know, they could have let the whole village know.