The next paragraph they wrote explains this: "Unlike 0 or "", nil does not have a default type. The default type of a constant is the type that it is implicitly

The next paragraph they wrote explains this: "Unlike 0 or "", nil does not have a default type. The default type of a constant is the type that it is implicitly converted to when a type is required. For example, "x := 0" declares a new value x with the value 0, but 0 is untyped. Untyped integer constants have a default type of int, so x is given the type int."