Substitute for 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.

You could try using arrowroot. This is a widely available alternative to cornstarch - it is used in cookery because it doesn't turn liquids cloudy like cornstarch does. In your case, it might work better as it also has a more neutral flavour.

Substitute 2 tbsps of arrowroot for 1 tbsp cornstarch, and make a slurry with cool water as you would cornstarch. One issue is that overheating can break down the arrowroot, preventing it from thickening, so you are probably better off adding it to the tea after removing it from the heat.