Hi Meghan, try adding the .csv extension:
tabcmd get "views/WeeklyReports/EmailSampleDetail.csv?FilterName=ABC" --filename CSVFilterDoesntWork.csv
The documentation says you don't need the extension for .pdf and.png if you use a filename parameter, but it's doesn't mention the rules for .csv, which appear to be different (I have no idea why). Good luck!
Thank you SO much Jeff! Totally glossed over that point in the documentation haha Appreciate your help with this :-)