Why baste bread with cornstarch?
I've seen bread recipes like the one that you described.

When bread is baked in an oven with steam- the starch in the crust is able to gelatinize before it all dries out and becomes crispy. This is what makes the crust crisp, shiny, and delicious- characteristic of "artisan" breads.

Most people don't have steam enhanced ovens (or the ability to hack their oven to add steam: How can I create steam in a normal oven to promote bread oven spring?)

The recipe you linked has water added for steam but then takes out extra insurance (they cheat) by adding the cornstarch glaze to mimic the effect. By adding extra, pure, starch on the surface of the loaf more gelatinization occurs. Additionally, cornstarch gelatinizes at a lower temperature that wheat starch. Conceivably, you should be able to use any starch and see similar results but, in the US at least, cornstarch is by far the most common.

It shouldn't be necessary if you are able to produce enough steam in your oven.