I found that observation to be a little flaky myself. Why use Christian? Why not Muslims, Hindus, or Atheists? Totally distracted me.
1. Muslims - - Hashashins
2. Hindus - - Hookas
3. Atheists - - they don't practice Rastafarianism
But, you forgot one...
Buddhists - - Nepalese hashish
Because Mark Twain lived 1835 to 1910, and through a lot of that period (all?), "Christian" was often just a synonym for "civilized," or "Western" or -- basically "people like us."
If you have ever read anything by Mark Twain or other writers of the era, you would know that it was common in the 19th Century to compare the actions of "heathen Chinese" and "savage Indians" with "Christian Americans."
There were so few Muslims, Hindus and Atheists in the U.S. at that time that no writer used those terms.
The language and the way of looking at things change. You cannot expect writers of XIX century to accommodate people living more than hundred years later.