So will building more muscle not boost your metabolism? Yes, but only by a small amount.

Then they go on to contradict themselves.

I'm not going to go find the paper, but studies have shown that the mitochondria of active people is very different than that of sedentary people. In sedentary people the mitochondria are clumped together and each cell has far fewer of them. They simply can not produce the same amount of ATP.

The article you are citing is the unscientific opinion of the author, who is confusing the difference in resting and exercising metabolic rate of the same person with the difference of resting rates of two different people.

I simply can't trust the opinion of someone who claims this is a myth because when you stop exercising your metabolism returns to its resting rate. That is simply not sound logic. Of course it returns to the resting rate, but that has nothing to do with how your resting rate compares to someone else who is in completely different shape than you.