In one way I have little sympathy. It’s the same as parents complaining after they show their child a violent anime, that it was a ‘cartoon’ and so it must be for children - having made that snap judgement without investigating the contents in the slightest.
On the other hand, as the article rightly suggests, there are established conventions in the publishing industry and this book defied them.
They are conventions I personally kinda hate, because they are the reason every Crime paperback looks the same as each other, and every Sci-Fi book is instantly recognisable as that genre on the shelves. But the conventions do exist.
In mass-market publishing terms, sparkly happy cartoon = children.
The publisher and author totally knew what they were doing here and they did it anyway. It’s wilfully misleading.
Whether established standards should be enough to absolve a parent of the responsibility to understand what they are giving to their child, though, you decide.