Hi @Minji
It's correct what you're reading as you can see on the KPN website.
Although it's not an EU country the same rates apply.
Tarieven per land
Wat is je bestemming?
Zwitserland: dezelfde tarieven als in Nederland
Jij kunt gewoon blijven internetten, bellen en sms’en uit je huidige abonnement zoals je gewend bent. Je hoeft geen Extra Bundel aan te zetten.
See here: https://www.kpn.com/mobiel-abonnement/bundels/buitenland/binnen-eu.htm
Google translate:
Switzerland: the same rates as in the Netherlands. You can continue to use the internet, call and text from your current subscription as you are used to. You do not need to activate an Extra Bundle.