Marshall has obviously researched the topic well as befits her academic background but as a reader it seems to me that Potter herself did acknowledge she was a fan of the original folk tales and created her own but I wouldn’t call it plagiarism.

It is also possible for folk tales of similar themes to develop independently. I doubt very much that the Japanese legend of Ishikawa Goemon was influenced by that of Robin Hood and yet they both became heroic outlaws of folklore. Indeed one only has to watch wild rodents in action to see where one might come up with a Brer or Peter Rabbit character.