Even if matter other than brains were capable of thinking, that does not ipso facto bestow the title 'thinking thing' onto things which are not brains. All we can say is "well, it is certainly not a brain!". That is it.

Also, computation is not cognition. We need to focus on what distinguishes cognition from computation, if indeed these are distinct. I feel we are using the wrong words and thus having unproductive conversations regarding this topic (in general).