This statement does not say that breastfeeding is only beneficial for six months. It says that breastmilk, alone, is sufficient for six months. After that other foods are to be introduced, in addition to breastmilk.
Gradual introduction of iron-enriched solid foods in the second half of the first year should complement the breast milk diet.
It is recommended that breastfeeding continue for at least 12 months, and thereafter for as long as mutually desired.
The benefits of breastfeeding do not suddenly disappear at six months.