This is only true if you subscribe to a dualist view of humanity, that is that the body is a separate thing from the mind. But there is also a monist way to look at it, that the body and the mind are one.
The totality of a human being can’t be separated into body and mind in a practical way. What about the senses which are both a bodily and a psychological function? What about emotions that are definitely felt both in the psyche and in the body? When I am tried I can’t think well, when I am depressed it’s harder to perform a bench press than when I am not depressed.
We are not just the brain. That’s a reductive way to look at human being. The ancient philosophers knew this, after all, mens sana in corpore sano.