As with most everything MMT, that central claim too is a load of crap. People got used to using bank notes for specie and then went to government schools where economic denialism is taught. Thus, they didn’t notice when the essential phrase about being redeemable in gold/silver started to be left off the notes and when the coins stopped being silver and became copper. The government imposed sales and exchange taxes on the use of other forms of money such as gold, silver and foreign money. That is what keeps people using U.S. fiat inflationary funny money. Absent the taxes on the sales and exchanges of gold, silver and foreign money, people could easily use other more stable forms of money and then exchange their good money for U.S. fiat funny money at the last possible moment to pay their U.S. taxes.
Note the lame to non-existent defense of their central claim by the MMT commenters.