What MMT says is really quite simple. Firstly, using the logic already noted, it points out that governments cannot go broke because they can always create the money required to pay their debts. In turn this means that they can always control interest rates. Then MMT suggests that the proper role of money is to serve the economy, and not wealth, so full employment should be the objective of an economy; until this is reached inflation cannot be created within an economy, except as a result of political or external shock that economic policy cannot control.
Given that this is a publication elsewhere this has to be taken as Ritchie’s considered opinion, not just fist typing.
Governments cannot go bust, can always control interest rates and inflation never arrives before full employment.
I’m not certain that reality agrees here.