I was stumped on this one when I saw it, but I was looking at the Tableau Public website this afternoon and it all made sense:
Freddie Mac must be using Tableau Public Premium. Hence the difficulty downloading the workbook. Why they chose to lock down the workbook but allow the data to be downloaded as an excel file is a mystery only they could answer.
If they don't want to share the only option may be to reverse engineer it. Was there a specific part that you'd like to replicate?