Technically, it is not, but the longest palindrome substring within that string is a string of length 1. In that case, any random letter in the original string is considered a palindrome on it's own.

https://en.wikipedia.org/wiki/Palindrome

A palidrome is a sequence of letter, numbers, etc that reads the same forwards as it does backwards. Therefore a 1 letter string can be said to be a "palindrome" because it reads the same if all the letters are reversed (in this case just one letter). From this we can deduct that any string will contain, at a minimum, a palindrome with a length of 1.