When you start MT, the basic guard everyone learns is the high guard. That means your shoulder are high, protecting your chin, and that your chin is already tucked in to look "under" the gloves.
Also unlike boxing, since kicks are a big part, and specially as a beginner, you usually look your opponent lower than just their shoulder. You need to be able to see movement coming from the hips. That will also make you tuck your shin.
So usually, you don't need to tuck it more than what your stance makes you.
Also, the high guard is very taxing to the shoulders, specially when you start to absorb strikes... So emphasis to relaxation is important. It also helps with you having a rhythm. Like superlek in the gif from CoolKnees.
As you get more experienced, and more relax, you will find your personal "proper" stance, depending on your style...
But if you gave him the impression that you where to stiff, telling you that you should not hide your chin could probably help you be more relax. Not only upstairs, but it will make you movement more fluid too.
