That is more or less correct, but it's a very weird situation. Scurvy had been known -- along with effective cures -- for thousands of years. The method of curing scurvy hasn't been secret since -- at the latest -- the Ebers papyrus of ~1500 BC, which correctly prescribed feeding the patient an onion. And in fact cures were widely known throughout the world since that time, including in Europe. Note again that James Lancaster had heard that lemons were effective against scurvy, 200 years before the Navy got around to requiring them.

From https://tudorblog.com/2012/06/16/voyage-of-the-scurvy/ :

> from about 1500 to 1800 two million sailors are estimated to have died from scurvy on expeditions to Asia, Africa and the New World.

> Why no cure? In Tudor England an effective treatment, scurvy grass (a corruption of cress), was commonly recommended; at the same time the Portuguese knew a cure, and so did the Spanish and the Dutch. For unknown reasons such wisdom was applied inconsistently (even by Britain’s Royal Navy after the Napoleonic Wars) until the identification of vitamin C in the 20th century.

It looks more like a case of the people making the decisions being unfortunately disconnected from the people who knew what scurvy was and how to deal with it. (And wilfully ignoring those who tried to point it out to them.)