I discovered that you can't send the schema name along with it. Even though it's stated in the documentation.
instead of:
setup.Tables["dbo.Tennants"].SyncDirection = SyncDirection.DownloadOnly;
the correct one:
setup.Tables["Tennants"].SyncDirection = SyncDirection.DownloadOnly;
Subtle, but the error didn't show this, I discovered it through the author's own answer.