I know that this is a year and a half later, but in case anyone else, like me, needs a sign-in/sign-out log put into Excel in order to run some employee functionality statistics.
As of right now, 3/3/2022, you can not save the audit log to XLSX. You can only print it, and IF you print it to a PDF, it prints as an image, and not as text, so if you try to then export from PDF to XLSX, the formatting is broken.
An interesting thing I found about the Audit Log is that you can change the rows from the default of 50 to 100, and the resultant screen is an HTML page. Just drag across the data and copy and paste it into a new XLSX sheet.