That's the thing; they aren't so structured that they're numbered in any specific rulebook that I'm aware of, but rather broad rules that should be taken at face value, like the one for anonymous sources - don't use them. There's no police to enforce it and for the most part you won't get into real trouble for breaking them, but to be respected as real news they need to be followed. But you don't have to believe an internet stranger. Here's David Poulson, the Assoc. Director for the Michigan State School of Journalism (start at about 4:05).
https://www.coursera.org/learn/gathering-the-news/lecture/fkFMk/rules-for-dealing-with-sources
Not using sources doesn't lend any credibility to what the author is trying to say. Why would I believe a story about the president's latest slight without a picture or a source I could double check? It places your belief in the story on the author's word, and not an actual fact.
So in the NYT example, if a large part of what they're writing is not citing sources then we can't conclude that the Times is writing honest articles that aren't 100% true. Why wouldn't someone cite their source? The reason might be legitimate to conceal their identity, but it doesn't help a story's credibility. Going a little further, if it's become more of a style rather than the exception over at the Times, the credibility of the whole paper is in question.
Edit:
I neglected to mention something that might be more clear. One of the first things journalists are taught to make a story. They need to be able to report 5 things for any story - Who, What, When, Where, Why, and How. An article that doesn't cite it's sources can't even get past the first one.