That's even worse, trusting someone elses word about the source. Trusting SauceNao code is one thing, but a random claim about the source is another. Why not just .. write your source in the source field? help:image_source definitely doesn't say that you should believe random claims about sources on the internet, and write that in the source field.
Also boorus and their users lie about sources, and modify the images all the time. Malice, mistakes, incompetence, and such. For example in this thread it is straight up recommended to manufacture a fake source. (Which may be the correct source, even may be provably the correct source. I think SauceNao gives more information for registered users about their data, and the matches.)
I don't see much advantage in copying source fields from other boorus. A disadvantage is that it strengthens misinformation.