It is true. I would like to qualify it by saying "At the beginning, the editor is not your friend." A friendship, even very strong one, can be developed along the time, based on mutual respect and appreciation, temperament match, etc. Even then, a good editor will still be professional and be honest when they edit the writer's draft.
A good book may not be a "successful" book, especially in literary fiction, since the post talks about novels. The reverse can be true as well. It happens often that the editor tells the author it is a good book, and the publisher allocates resources to marketing it, schedules book tours, etc. while at the same time both the writer's agent and the editor/publisher expect the book may sell only ~2,000 copies. The target audience are expected to be other writers, a subset of avid literature readers, etc. They don't expect it will earn back the advance paid to the author. The publishers have portfolios and long term visions. Of course, this doesn't apply to small publishers, most of which cannot afford it.