A source from the counterfeit detection facebook group mentioned that this test once was intended as a valid check for rebacks, the glue will start to bubble if microwaved and the reback will fall apart.
I only remember this test as a myth from various magic internet forums where people claimed that some counterfeits might catch fire and burn if microwaved shortly. This might be true for some which are covered with foiling but it is far from a real world test.
Real magic cards will survive this but every foil will instantly catch fire and might even damage your microwave if heated.
Overall this test belongs to the category “not recommended” and better don’t even try it.