MMT would claim rather simply that the debt can be paid back by printing more money. More money in the system means more people are employed. If you have full employment, more money leads to inflation and to fight inflation governments need to raise taxes, to take money out of the system. Obviously taxes can not be raised to infinity.
So no MMT does not claim the US can just borrow money without long term costs.
They are planning to buy or maybe just electricity from a new reactor type, which has never been built, starting 2030, when conventional nuclear power plants, with known technology take twice as long to built.
So if they stop the project in 18months, it has just been a waste of money.