The Universal Model doesn’t prove that it was Sedna specifically that passed by Earth. It simply uses Sedna as an example to help illustrate a point regarding the mechanism(s) responsible for the flood that did happen 4300 years ago. To be clear, evidences for the year of the flood are given in more detail in Volume II of the Universal Model.
To find out more regarding what the Universal Model says about Sedna, start reading in Volume I of the Universal Model on page 495 and go through page 496. You can reference note 8.3b for more information as to the research as well in the Notes section of the book.