Mung beans can be cooked without soaking them first.
Since mung beans are smaller beans they cook faster than other beans so really no soaking is needed or required before cooking mung beans.
However if you do soak the mung beans in water before cooking the mung beans then there will be less gas in your stomach from eating the mung beans.
Soaking the mung beans and other beans helps remove gas producing enzymes in the beans.
But you can easily cook the mung beans without soaking them.
I've always just cooked mung beans without soaking them first and they cook just fine.