You are taking the quote out of context. Here is the complete text from the Story of Civilization:
Twelve years he wandered, imbibing wisdom from every source, sitting
at every shrine, tasting every creed. Some would have it that he went
to Judea and was moulded for a while by the tradition of the almost
socialistic prophets; and even that he found his way to the banks of
the Ganges, and learned the mystic meditations of the Hindus. We do
The Durants are just repeating what it says in many articles and encyclopedias such as the 1885 article from "The Disciples of Christ" by JW Lowber, where it says: "Plato sojourned for a time in Egypt; and some think that he also visited Palestine, Babylon, Persia and India."
These legends are based on dark age Arab accounts of Plato which make these claims and were later repeated by medieval scholars.