I think the first implied question is whether the use of foil in the microwave is safe. Per the USDA,
[...] small pieces of aluminum foil can be used to "shield" areas
of foods, such as poultry drumsticks and wings, to prevent
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.
As to the question of whether there is good reason to use the method in this specific case, in the linked recipe, the foil is on top of the cup with holes poked in it. It is not in contact with the food as far as I can tell. The author gives no reasoning or background on why foil is used instead of, for example, plastic wrap or parchment with holes similarly poked into it. I suspect it is to slow the heating of the custard at the top surface (microwaves will not penetrate the foil), while still permitting steam to escape slowly so as to not water-log the custard. Steam in contact with the surface will also help cook the custard at the surface.
Still, the presentation of the recipe (including egregious punning) and lack of explanation don't leave me with any sense of why it would be needed or effective in this particular recipe.
Are you willing to take this risk with your microwave? If so, go ahead and try it... Truthfully, my best guess is that the foil makes little difference compared to say using plastic wrap to slow the escaping of steam and therefore help cook the top, since the microwaves will still penetrate the custard from the sides and bottom, but I have not experimented.
My own best guess is that plastic wrap, with holes poked in it to allow the slow escape of steam, would be equally effective.