I belive this summary above from a user might desribe it very well, till a better definition comes along.
The tag looks like it's required for "project contains content produced by generative AI tools," not "generative AI tools were used in the production of this project."
So if you trace ai references, the content is not ai made. Even though other artists might give you the same stinky eye they would give you for tracing their art.
If you created the content with a prompt and hand edit it afterwards, it fits the definition in the initial post and that metainfo box.
If you wrote a story and spell and grammer check it with ai, you still wrote it. If you prompt a llm to give you a story and you fix some plotholes, it also fits the definition of gen ai in the initial post.
And the stance I read between and in the lines is: it's just a tag and only important for assets. And if you have to ask if your prompt made asset is gen ai, then it is.
As for percentages, imagine you take an image from an IP. Like a famous cartoon rodent. Then you modify the picture. How much do you have to modify it, for the new image to no longer be a legal problem if you would publish it as your own creation? If there is a clear answer to that, I assume it could apply to gen aI as well.