They managed to convince themselves via a combination of wishful thinking and basic economic illiteracy - that's all there is to it.

MMT DOES make sense, as long as you are careful to qualify who it's making sense for. If you are a congress person with a very short term time horizon MMT makes a whole bunch of sense. Incidentally, our country is run by congress people with very short term time horizons.

If you are anybody else besides maybe a defense or infrastructure contractor MMT does not make any sense.