typewriter uses Atom's
Settings > Editor > Preferred Line Length setting to set the width of the centered column.
By default it works for (GF-)Markdown, Latex and MediaWiki files. You can change what file scopes enable the typewriter plugin in the settings or have it enabled for all files scopes.
Inspired by Zen
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.