Since travertine is a chemical precipitate, I presume the fractionation of isotopes between solid phase, thermal fluid, and atmosphere ceased when the rock lithified, which was probably shortly (geologically speaking) shortly after the juveniles made the prints. That established the radiometric clock.
The researchers most likely used the same approach as the dating of archaic human footprints in Africa. The Laetoli footprints were made by two Australopithecus hominids walking on fresh wet volcanic ash. Once the ash lithified (which would have been shortly after the footprints were made) the isotope ratios were frozen in place, giving a radiometric date of 3.2 million years for the Laetoli footprints.
I am not interested in investing the time to read the technical paper the handprints researchers published with all the technical specs of the study , but if this is really bothering a bible thumper, I am sure the scientists would email anyone a print of their paper.