Even a moron can write directions for how to milk a cow. It doesn't matter how poor the writing is, it matters whether a person understands it. People can (and do, regularly) understand poor writing. Even a child can write its parent a note expressing love in terrible writing that the parent will understand, and the idea remains right and true regardless of the quality or style of writing.
Historical recipes are terrible because they lack details. They don't tell you to add salt, they don't include accurate measurements (of temperature, time, ingredients, etc), they don't explain methods. Yet people of the time they were written can still follow those recipes and cook the intended dish. Writers always leave out details that are assumed by the reader. At the time those recipes were written, those people reading them would have already known all the left-out details.
If I write you a book on programming, I'm not going to explain to you what a computer is, how it works, how to copy software onto it, how to power it. You're going to already understand all that, or you probably wouldn't be reading a book on programming. Depending on what you understand, and depending on what ideas I try to convey, and how I do that, may change the end result of the information you come to understand. But they don't make the ideas less or more valid because I wrote it this way or that way.
Writing is only the communication of an idea from an author to a reader. Style, form, method, construction, etc are all inherent to writing. But the validity of the expressed idea is not. The content of the idea is completely separate from the "art" of writing. A painting of a blue sun does not affect the sun, nor does a painter's ability to depict the sun affect the sun. But a bad painter can still paint a bad painting that a viewer will understand to be the sun.
"Good writing" is like "Good art". It's subjective. As long as the recipient got what the producer intended, it's good enough.
Historical recipes are terrible because they lack details. They don't tell you to add salt, they don't include accurate measurements (of temperature, time, ingredients, etc), they don't explain methods. Yet people of the time they were written can still follow those recipes and cook the intended dish. Writers always leave out details that are assumed by the reader. At the time those recipes were written, those people reading them would have already known all the left-out details.
If I write you a book on programming, I'm not going to explain to you what a computer is, how it works, how to copy software onto it, how to power it. You're going to already understand all that, or you probably wouldn't be reading a book on programming. Depending on what you understand, and depending on what ideas I try to convey, and how I do that, may change the end result of the information you come to understand. But they don't make the ideas less or more valid because I wrote it this way or that way.
Writing is only the communication of an idea from an author to a reader. Style, form, method, construction, etc are all inherent to writing. But the validity of the expressed idea is not. The content of the idea is completely separate from the "art" of writing. A painting of a blue sun does not affect the sun, nor does a painter's ability to depict the sun affect the sun. But a bad painter can still paint a bad painting that a viewer will understand to be the sun.
"Good writing" is like "Good art". It's subjective. As long as the recipient got what the producer intended, it's good enough.