Earlier quoted context omitted.
A neural net is not a database where the original source is sitting somewhere in an obvious place with a reference. A neural net is a black box of functions that have been automatically fit to the training data. There is no way to know what sources have been memorized vs which have made their mark by affecting other types of functions in the neural net.
Presumably, if a passage of any significant length is cited verbatim (or almost verbatim), there would have been a way to track that source through the weights. The issue of replicating a style is probably more difficult.
Figure this out and you get to choose which AI lab you want to make seven figures at. It's a really difficult problem.