That’s not free money; it’s money taken from people under threat of violence.
This is not correct. If you try minting coins, you can be shut down. You can buy and sell using whatever you want, yes. There aren't laws against using alternative currencies. But there very much are laws against creating them.
Grandparent comment is referring to the position that taxes are extracted via threat of force. This is an extremely emotionally fraught position, and it's difficult to discuss rationally.
So, the reasoning goes, if you have to tax to print money, it is done by resort to force or threat thereof.