Only AOW (state pension) is a requirement by law and for full-time salaried employees there is no way around that. Private pension (what is meant here) is not required by law though financially very lucrative compared to doing it yourself. It can be made required by making a CAO (collective bargaining agreement) "algemeen bindend" for all employees in a sector/company but this is not always the case in all sectors (e.g. IT). Note that this is way more complex and this is just a simplification.