I don't think that anyone is trying to say that mind is matter. Even those who are of the view that mind arises in the brain do not think that the brain is the mind....just like the people who are of the view that sight arises in the eye do not think that the eye is sight.
I guess it depends on what is meant by "base". Some people are of the view that there are six senses and for instance to consider the eye they think "yes, I have two of those which reside in sockets in the front of my head. For these people the "base" for sight resides in those two lumps of matter which reside in the front of their head. These people have no question about where the "bases" for hearing, smell, taste, and touch reside because the suttas describe them with explicit body parts indicated as the base. Some of these people ask "where is the base for the mind?".....and for me the logical body part which would be used as the base for the mind is the brain......this seems like a no brainer to me.
What I have just said is how some people relate to the senses. Other people view them completely differently. Some people say that the buddha never talks about the lump of matter called the eye but rather he is talking about how we perceive that vision arises.....they consider the eye to be not the thing which gives rise to sight but it is the concept that we use to understand how sight arises....it is a fabrication...it is an explanation of how sight happens. Some of these people will say that if you think that sight arises in the eye then you are making a delusional self called "the eye" and giving it properties. Some of these same people will say that it is better that when there is sight to just see what is seen and not to fabricate a seer or a seen. For some of these people none of the six sense bases are matter....they are mental fabrications.