How could Leitha's identity be forgotten?

When Leitha was rediscovered, had people simply forgotten what the vessel originally was?

Yes. After conversion to river-barge use, her original identity and purpose were largely forgotten.

The ship survived physically while knowledge of her original naval identity faded. Her adaptation as a river barge concealed the silhouette and equipment that had marked her as a monitor. Investigators therefore had to reconstruct the hull's history through forensic engineering rather than rely on an uninterrupted museum provenance. The episode shows how commercial reuse can preserve a warship's fabric even while erasing public knowledge of what it was.