Of course a government can write as many IOUs as its wants, and it can create laws to say that people have to accept them. But if the US government were to issue a new $1 quintillion of US Notes tomorrow, they will effectively be worthless, especially if people suspect it will issue $10 quintillion the following day.
I don’t see your point. No MMT economist would disagree with this. MMT economists take inflation seriously because that is the main ‘constraint’ on government spending.