Is it wise to use an aluminum foil in the microwave? Is there a substitute for the aluminum foil?
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.