If the flat stools are a new symptom, get another scope. No diagnostic procedure is perfect and sometimes things are missed. If the doc was in a hurry with a pre-conceived notion, it's possible. It's also possible the diverticulosis has become diverticulitis. In any case, I'd not delay.
Diagnosed at age 47 after bleeding for many years
Stage II, T3N0M0
Sigmoid resection 8-20-02, 6 mo 5fu/leucovoran