I had sigmoid colon SRS recently and can confirm the lubrication. The reason why anal sex requires lubrication is because when you insert something into the anus, you are not inserting it into the sigmoid colon. You are inserting it into the rectum. The rectum is about 12 centimeters (4.7 in) long, and there is a sharp angle before it turns into the sigmoid colon. I don't think the rectum produces much lubrication, if any.
If you look up diagrams of the rectum, sigmoid colon, and the overall colon, you will get a better idea.