Brains are material. Neurons are material. Thoughts are electrical impulses in the brain. Without brains, neurons, electrical impulses etc. etc. there are no thoughts and there is no mind.
The materialist stance is not that 'brain' and 'mind' are synonyms but rather that minds have a very material basis - Namely brains, electrical impulses, neurons and all the rest of it. And that without the material there is no mind.
Do you think minds can exist independently of brains (or some other material equivalent)? If you do you are a dualist. If you don't you are a materialist.