There's a huge difference between: - Don't use this, it's harmful to you, and - Don't use this, it won't make things actively better.
Study after study has shown that it is extremely, almost absurdly difficult to lose weight and keep it that way in the long term.
If "not actively causing long term weight loss" is the only criteria for advising against something, then we should be advising against a heck of a lot more things that are perfectly fine.
It's phrasing like this that causes people to go on crusades against random foods, rather than actually using moderation in all things.