Harvest was glassed in October 2525. It's possible, though I'm not finding any corroboration on the Halo Nation wiki, that the public was not told about the existence of the Covenant until some time after the events of H4: FUD.
ONI unveiled the existence of SPARTAN-II supersoldiers in 2547, twenty-two years after the start of the Human-Covenant War and five years prior to the war's end. Until that point, general "knowledge" of SPARTANs was rumor and conjecture. The existence of SPARTAN-III soldiers remained classified.