You don't even need a scientific breakthrough or difficult bit of engineering to successfully use that strategy. Coca-Cola is one of the more famous cases of a company that chose to keep its "invention" secret rather than disclose it with patent filings. It was common at the time to patent the formulations of "patent medicines", and the 1890s version of Coke was indeed patented, but later formulations were deliberately not patented, in order to keep them secret.
