This is not a UBI. Adults who were not of retirement age who were able to work would have to work to be paid it. Every adult who wanted to work would be guaranteed a job. So labor would be the source of this money’s ‘value’ in that ultimate sense.

“Fiat” money is not fake money. It is legal tender. It is what government has said can be required as payment (though people are free to make exchanges using whatever both sides agree is good with them). It has always existed. Back when gold (silver, etc.) coins could be required as payment that was the legal tender.