That “open network” is a bit misleading, because in this case, the network is still owned by Telia, which leases it to other service providers. These providers, in turn, pay Telia rent for using the network if they get a customer to take their subscription.
So, in practice, because of this rent, the prices of other service providers rise to such an extent that, from the customer’s perspective, the subscription is likely cheapest to get from Telia in the end.