Please link to your source of information.Dr Mew is an absolute scammer who had surgery in his jaw and still looks like an ugly sexless rat, because looks are about harmony, not one specific feature.
And his method is some dangerous cope that can cause irreversible damage.
I don't think that a tongue exercise is harmful.
There is a tongue swallow syndrome. The tongue pushes the teeth during swallowing, even bending the teeth forward...