I am not saying that is necessarily wrong (maybe it is), nor that is not useful, but it just seems like another small step in removing the individuality of the people working there. It doesn't feel right to me.
The writing advice that I follow and that I like to give is that your writing style should reflect your personality. Your individuality should be with you on some level even if you are representing your company in a meeting with a client, or to the press, or internally. Writing style should be unique to you, not to your company. An exception should be someone whose job is to be the spokesperson or a lawyer creating a document that declares the position of the company. But that's a very different use case from the one declared at this guide.
The way it says "How we write", "Our voice is who we are", "We are clear/honest/transparent", "We are empowering"; it looks uncannily familiar to the Borg.
eidt: Another thing. When they say "Swap formal words for normal ones. There's a whole lexicon of British "business-speak" that one can use to sound professional, but also needlessly distant. We try to avoid that:
It reminds of another cultural change where the principles remained just the same, like: "Remember when everybody on business had to wear a suit and people would make fun of anyone who showed to work on a plain t-shirt? We are not like that. Here we make fun of people who want to wear a suit".