The comparison is completely fair; I don't think he's making the general statement that Go is faster than Java. He can legitimately make the claim that the overhead of a slice of ints is far smaller than an array of ints in Java.
You can add methods to an int in Go without any overhead by simply defining your own type that is backed by an int and then adding functions to that type. It still costs same.
Java performance for real-world, long-running, well written applications is greatly enhanced by an excellent JIT that improved performance over time. Go can be faster for looping over a list of integers while Java can win at branch prediction in complex code paths.