Not quite true. While I believe Secretariat set the record in all three of his Triple Crown wins, IIRC in his Preakness the official clock broke. Some folks hand timing him said he broke the record, but I think the official decision has him a tick off it. And I agree, Secretariat in the Triple Crown was the most dominating athletic performance of my life by any species. The Triple Crown winners that followed him seem lackluster by comparison.
That has all been officially corrected and he does hold the record at the Preakness.