It is appearing not in the static if, but in the is expression, which I described further in the rest of my first reply. Sorry if that wasn't clear.

The other template syntax - template foo(alias T) can take as T any symbol, not just a type.

It comes from you, the programmer. Like I said before, is(T == U[], U) means "is T an array of some type, the type which I (the programmer) would like to refer to as U?". That's all there is to it (well, not quite, but it should suffice for starters). You're simply introducing an identifier. So, when T is an int[][][], naturally, U becomes an alias to int[][] (look at the converse - when U is int[][], U[] is naturally an int[][][]).

You can think of that test as this:

import std.traits : isDynamicArray;

// ...

static if (isDynamicArray!T)
{
    alias U = typeof(T.init[0]);
    // ...
}

...which would roughly be the same thing - you test if T is a dynamic array of some type, and then make an alias for that array's element type. It's just that the is expression allows you to create such alias in situ.