tabcmd is a Server tool. You can't run it against a Desktop file.
You're going to need to do it against Server, though, and not Desktop
We are looking for a way to avoid having to publish ever workbook to the Tableau Server because our site doesn't have enough space and I don't think we will be successful requesting more space be added. For example a single data source file is over 10GB.
I don't believe there is anything in the tableau server license conditions that limits the size of your vizzes.