Then there is the mundane possibility that some more recent traveller brought the tablets over to the States...
England started systematically collecting cuneiform tablets in the mid-19th century. One early collection was in 1849. The peak years of collection were between 1850 and 1920.
Chief Joseph lived from 1840 to 1904.
So it is more likely that some intrepid English explorer picked up the tablets from over there and brought a few of them to the States.
Seems more reasonable than someone schlepping a receipt for a couple of goats around for thousands of years.
It is also possible that Gloria Farley was right in her book about all kinds of ancient inscriptions when she collected them from rocks visible from some of our larger rivers.
Perhaps SunkenCiv has the book title. Merry Christmas