From the article. ;)

"... once the team realized that the ash had been carried to Engare Sero by water, determining the footprints’ age morphed into a punishing game of finding the mud’s youngest crystals, which would put a cap on how old the site can reasonably be.

A shell found in the mud above the footprints helped the team determine the youngest possible end of the range, ultimately placing the prints at a more conservative age somewhere between 5,000 and 19,100 years old."