Some artists limit their focus to a single medium intentionally. It doesn't imply that they are incapable of using other media, it may be as simple as the artist found a way of expressing ideas and concepts in a single medium in order to find an unique voice.
I can play a lot of instruments in a range of styles, but I've found my voice on a resonator guitar. It doesn't mean I can't use anything else, it means that I prefer this medium and voice.
I don't think that preferring to use one medium over another means that person is close minded at all. It just means that's what they prefer.