Foil in microwave

When metal is exposed to microwave radiation, an electric potential difference can develop as the microwaves generate electric charge in parts of the metal. Flowing electricity can cause sparks as electrons migrate to places of lower potential. Solid metal is susceptible to this because its electrons are relatively loose, making it a good conductor of electricity.

Calcium (Ca) in milk is bonded with other atoms (mostly as calcium phosphate), so it doesn't behave as a solid metal would: first because its electrons are secured in bonds with other atoms, and second, because the Ca atoms are not aggregated together but intermixed with nonconducting ones.

See this wikipedia page, it states that if not crumpled aluminum foils is generally safe, so make sure you do not crumple it. Then it says

The USDA recommends that aluminum foil used as a partial food shield in microwave cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.

The reference states

However, small pieces of aluminum foil can be used to “shield” areas of foods, such as poultry drumsticks and wings, to prevent overcooking.

It also states

General Rules for Safe Use of Aluminum Foil:

• Use new, smooth foil only. Wrinkled foil can cause arcing (sparks).
• Cover no more than 1/4 of the food with foil.
• Shape the foil smoothly to the food so no edges stick out
• It makes no difference which side of foil (shiny or dull) is facing out.
• Do not place the foil closer than one inch from the oven walls.
• If the microwave oven has metal shelves OR a metal turntable, don’t microwave food in foil containers or metal pans, and don’t let foil used for shielding touch or be close to the shelves or turntable.
• If you see arcing (sparks), immediately remove the foil shielding; transfer frozen food from foil container to a microwave-safe utensil.

My bolding.

So make sure you only cover the top of the cup and make the foil smooth, do not crumple it and it should be safe.

The reason to use this as stated in the blog is speed, a few minutes compared to much longer.