[rlmw]

Because he takes a reference to a char[] in the constructor without copying it. char[]s are mutable so this means that the internal state of his String can be mutated by something else without any synchronisation guarantees.

[MichaelGG]

But that's nothing to do with thread saftey - even with a single thread if the char[] changes then stuff blows up right?

[sgustard]

Correct, and Java's String(char[]) makes a copy of the array for that reason. This article's benchmark gets a boost by avoiding that.