All of that can absolutely be to reduce bias; there doesn't have to be some sinister motive. FWIW, the publication issuing these criticisms is heavily left-biased. Essentially, The Intercept is criticizing another publication for not putting enough bias into their reporting. I would consider that to not be good practice.
One of the main way biases are determined is by what words are chosen to describe a particular situation. From MBFC: "They [publications] may utilize strong loaded words (wording that attempts to influence an audience by using an appeal to emotion or stereotypes),"
All the memo is doing is setting the tone to keep from introducing strong biases. Again, no sinister motive and just sticking to the facts instead of appealing to emotion.
Can't they cite the UN now?
I'm sure they absolutely can if quoting someone from there; no need to jump to conclusions. Style guides are meant to reduce the amount of author / publication bias or otherwise provide a consistent tone among different authors under a particular publication.
Again, I have not seen this particular memo in whole (just the parts that this article cherry picks to make their own point), but I'm aware such guidelines exist and are common.