Humans have always found ways to consume their labour in the future, it's not just a fiat construct.

From salting meat to storing harvested fruits, grains & honey in jars.

Money is an abstraction of value. An individual can trade a consumable good now for this abstracted value. There's an assumption (speculation) by the producer that this abstracted value will be accepted in the future.

Oil is priced in USD but oil is not fiat. Choosing to store oil for the future is not fiat, but it is speculating. Humans are always speculating. We're constantly assessing where we allocate our time, labour & resources for our greatest benefit. This has occurred long before fiat currencies were forced onto is.

Remember that the defining feature of fiat is that it is imposed by decree. While powerful people can force us to use their tokens in certain situations (ie to pay tribute/taxes), they can't force us to value it as individuals.