Thanks for the name. I skimmed through it and tbh, I'm not seeing why no additional context/doubt could be provided. They might have internal guidelines to not do so, but the charter itself does not seem to stop journalists from providing additional context outside of official statements/reports. It seems to me that this sentence was a choice by the editor/journalist.
I just searched for some keywords, "fact" landed me on the paragraph that seemed most applicable: "duly accurate and impartial news, current affairs and factual programming". The other paragraphs I found, that could have been applicable, were about being impartial when UK politics were involved.