If a sigmoidoscopy scope shown no polyps or cancer in the area tested, does that mean there's no cancer throughout all of the colon?

Not really. Sigmoidoscopy looks at the sigmoid colon. However there is still that possibility that there may be something in the say cecum for which colonoscopy is a good way to check out that region.
No. A sigmoidoscopy allows viewing of the rectum, sigmoid colon and occasionally the last part of the descending colon. This is about 25% of the colon and leaves 75% of colon unexamined and at theoretical risk for cancer.