If you make a painting now, it wouldn’t be based on those thousands and thousands of paintings since, although you have seen them, you apparently do not remember them. But, if you did, and you made a painting based on one, and did not acknowledge it, you would indeed be a bad artist.
The bad part about using the art of the past is not copying. The problem is plagiarism.
The author mentions that some of the changes broke things, but it’s a long way into the article before the word “test” appears. It’s only point 6/7 of his recommendations.
Making changes with no test coverage is not refactoring. It’s just rewriting. Start there.