They were not just “eventually admitted,” but listed on the ship’s manifest. There was no secret.
“The British public were not told at the time about the 5,000 cases of small arms cartridges that had been aboard but were deemed non-military.”
https://www.theguardian.com/world/2014/may/01/lusitania-salvage-warning-munitions-1982
Now anything recovered beyond the stated items is another matter. Divers claimed to have found both aluminum powder and bronze powder, both used in the manufacture of munitions, but I’m not sure even those would count since they are basically raw materials. Claims of recovering gun cotton might be different, though again I’m nt sure if that would early be classified as “munitions.”
"The nearly century-old debate about whether the passenger liner Lusitania was transporting British war munitions when torpedoed by a German U-boat is over. Physical evidence of just such a cargo has been recovered from the wreck, which rests 12 miles off the Irish coast in 300 feet of murky, turbulent water....Military ammunition is considered war munitions.
"This past September, Bemis's team used a remotely operated vehicle to penetrate the wreck. They were able to clearly identify a vast amount of ammunition in an area of Lusitania not believed to have carried cargo. The Remington .303 caliber bullets the team discovered on the ship had been used by the British military during World War I."