Except that idea is simply historically untrue. Patent were started primarily as a way to modernize the guild monopolies that were common place at the time, but were unsuited given the greater levels of globalization and international trade.
The idea that they were meant to serve as a way to reduce trade secrets falls flat in the face of the fact that most places with strong patent and copyright law, also have strong laws protecting trade secrets
You are correct when you are talking about the origin of British patent law. There a king could grant a patent for a wide range of monopolies, including practicing a particular craft only by guild members, etc.
The US patent law, which I referred to, is not as old, and it definitely talked only about inventions, and explicitly required drawings, explanations, etc to be made available: [1]
Given that american patent law was created originally as a copy of british patent law, through the statute of Anne, that distinction is irrelevant or even misleading.
The idea that they were meant to serve as a way to reduce trade secrets falls flat in the face of the fact that most places with strong patent and copyright law, also have strong laws protecting trade secrets