Not necessarily, the challenge is to draw with "ink". Inks, ink pens, brush pens and markers can come in a variety of values. Even if you use one colour, some of them go darker with more and more layers added on top. Also, inking with a paintbrush can also result in a range of values.

I did a fair bit of hatching and stippling with a black ink pen in high school, but never tried the other stuff above, so last year's Inktober I had a lot of fun playing around with those techniques as well.

You can see different interpretations of Inktober in the Inktober tag at Tumblr. But giving yourself a limit of just black and white is definitely a good way to challenge and develop drawing skills. :)