This story has already been talked about in 2020. Tatler and KP were in conflict for months over this article but let's not try to rewrite history.
I actually read the original article and the problem with the article wasn't just about the "Kate's thin" comment.
The article was called "Catherine the Great" but it was anything but great for Kate. A lot of the article was focused on pitting her against Meghan. It was Kate the great vs Meghan the evil manipulator.
Then, it painted a negative picture of Kate's family basically calling Carole a social-climbing snob, Pippa fake and Kate being bland (which was actually a praise in the article because it made her assimilation into the RF easier). It is implied that Kate is faking her accent to sound more posh
"The Middletons will always have close ranks. None of them can quite believe that they have hit the jackpot."
The article also said that William has a weird obsession with Carole: "She's the mummy he always wanted"
The article then basically paints Kate as a manipulator, the ultimate power player in the family.
"She has a ruthless survival streak, just like the House of Windsor. It's why she is so well-suited. She keeps her head down because the price of being Queen is so great".
Hilary Mantel's comment about Kate being a plastic princess is mentioned
It also mentioned Rose in the article and her falling out with Kate due to Rose's ‘apparent closeness to William’ (Tatler also has several articles on their website about Rose and the affair rumours but I thought evil William and KP forbade the media from talking about it. Last time I checked, Tatler is a British publication).
The article also mentions Kate's weight, saying that "Kate has become perilously thin, just like – some point out – Princes Diana" which basically gave credit to the rumours that have been out there for years that she has an eating disorder
The article then quotes one of her "friends" (we don't know who it is) saying that “Kate is furious about the larger workload. Of course she’s smiling and dressing appropriately but she doesn’t want this. She feels exhausted and trapped. She’s working as hard as a top CEO, who has to be wheeled out all the time, without the benefits of boundaries and plenty of holidays.”
"Kate and William wanted to be hands-on parents and the Sussexes have effectively thrown their 3 children under the bus"
After that, KP threatened to take legal action. They went back and forth for months. In the end, the article was edited