I’ve tried this twice. Both times I researched the improved ‘stone bounty’ tech and stormed the enemy with mass knights, igniting buildings, then surrendered. First time, I manually counted the stone accrued as it popped up above their head and stopped above 500 which didn’t work. My mental maths might be off so the second time I razed the entire base, still no award. Later I’ll try using the basic version of this tech.
Update: it worked with the basic version of the tech.