Confusing note on the custom shader instancing example | Bevy | Page 1

In the bevy instancing example there's a note about the instance index that I don't understand

// NOTE: Passing 0 as the instance_index to get_world_from_local() is a hack
// for this example as the instance_index builtin would map to the wrong
// index in the Mesh array. This index could be passed in via another
// uniform instead but it's unnecessary for the example.
out.clip_position = mesh_position_local_to_clip(
    get_world_from_local(0u),
    vec4<f32>(position, 1.0)
);

Why is the hack here necessary? It's not clear to me what would be wrong with using the built-in instance index

Instancing

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#Confusing note on the custom shader instancing example