We know from Lord of Souls that Orsinium was sacked in the early 4E by a combined invasion from Sentinel and Wayrest. Only the intervention of an imperial legion prevented the slaughter of non-combatants (to hear an orcish legionary tell it, anyway.) Supposedly most survivors went to Skyrim, but if you play an orc at Helgen, one of the legionaries tries to console you that they'll make sure your body is sent back to Orsinium. So what's going on there in 4E200?

I haven't read the offending quote, but sacked does not necessarily mean destroyed. The Great War saw the sack of the Imperial City and White Gold Tower, but by all accounts, they're still standing.

The Empire seems to have re-exerted control over High Rock and Hammerfell in the ancient times, and I'll bet that to keep the Orcs on their side, the Medes were clever enough to broker peace between the Bretons, Redguards, and Orcs again, and return Orsinium to them.