We define the word habitable to be the ability for a planet to have life on it. But that is by standards of Oxygen/Nitrogen breathing, H20 drinking lifeforms. Who's to say that there aren't carbon life forms that have evolved to live off elements that are toxic to us
We have those life forms here already=) Microbes that eat arsenic. I am sure there are a lot more examples in extreme areas of our planet if there was a bigger budget to find them.
