Fiber is not laxitive. That's correct.
HerbaLife tablets and capsules don't consist exclusively of fiber. They contain additional ingredients, of which I am sure they act as a stimulant within the digestive system. Meaning an increased frequency of colon contractions, similar to coffee. Coffee is not a declared laxitive either, yet everybody knows what coffee does ... ehem, in the morning
I remember there were also tablets in higher concentration containing Guarana among other ingredients, which is a high potency caffeine substance. Actually way better than coffee.
I am sure HerbaLife has changed their Formulas during the last 15 years.
And yes, you can accomplish even better results for less by browsing health food stores etc.