I think you misunderstand the author’s job here. This is a book that is, in some of the passages that have been printed, sets out what other people have told him happened at various points in time. He is recording their firsthand accounts, not himself establishing what was true or not. It’s not a judgement.
So in the VF instance, he reports what he was told by the author of the story - that what they found out about Meghan’s charity work as at the date of the interview meant they did not feel comfortable about presenting her as a philanthropist as she wished. She had, at the time, spoken at a couple of North American events and taken one trip to Rwanda earlier the same year. That, presumably, did not convince the team at VF that it would be appropriate to describe her as a philanthropist, so she was presented as an actress with a strong social conscience (“One of the strongest bonds Prince Harry and Markle share is their philanthropy” the article did say, without going into details).
Tom Bower is under no duty to challenge Sam Karshner on this, anymore than Oprah Winfrey was under a duty to challenge Meghan on some of the ridiculous things she said in her interview. Oh, and when Tom Bower finds conflicting versions of a story - who approached whom about Meghan guest-editing Vogue, for example - he has included both versions, and lets the reader decide.