If it’s the cornstarch in the custard powder that’s doing the necessary work, why not just use cornstarch?

You could certainly do just that, which is part of the author's note at the start of the recipe. The other things the custard powder brings with it are vanilla flavoring and a yellow coloring, from the annatto- which you don't get in cornstarch obviously. You would want to add in vanilla to make up for that. Your finished cookies will not have quite the same color as those made with custard powder or vanilla pudding mix, but that won't affect the final taste. I have no idea why the author felt the need to use either product, actually- unless he just happened to have it occupying shelf space he wanted to free up. I wouldn't go to any trouble to specifically obtain either one to use myself, for exactly the reason you give. It isn't bringing anything particularly special to the mix.