Thinset and grout are not the same. They are formulated
only for their specific purpose. If you had a few small
joints to fill, such as in a small repair, you could use the
thinset as grout.
On "very narrow joints" as you describe, you shouldn't
use sanded material, as thinset is. The grout also has
properties that make it set-up quickly, so it can be worked
to it's finished look. Thinset is designed to do the opposite-
stay fluid for a long time, thereby washing-out the joint as
you sponge it.
Spend a few bucks and get the correct grout and save
yourself some work. Do it right.