It’s likely that the coins were just used as payment along a chain of buyers and sellers until they reached there.
Being made from real precious metals, they still had value no matter where they were used even if they came from some weird place that the final holder had never heard of.
In those days you could assess the value of a coin by weighing it. That facilitated trade, because they were using genuine money.
Most 4th century Roman coins weren’t made from precious metals (with the exception of the gold used to pay the army). The article says these were copper and bronze. Smaller and thiner than a dime. The Romans did “wash” them in silver to make them look more valuable than they really were. Kind of like when the silver disappeared from US coins in 1965.