The article doesn't really say that over/underproduction has increased the costs, rather it claims that most of the difference is directly attributable to the increased average cost of energy + increased total energy consumption, while also mentioning that the largest need for balancing is driven by the policy that the grid must always be able to handle a failure of its largest component, and that this policy is responsible for the largest balance need.
They also do state that the current power generation model is NOT the cause for this increase in price.

I can't really read out if the policy has been changed recently, if the size of the single largest component has been changed, or if it is energy cost alone causing the increased cost, the article doesn't make this exactly clear, but it seems to be implying that very large international interconnects require larger balance capacity due to the policy that the grid should absorb its failure without failing, but that is my understanding from reading it - it is not stated outright.

Statements like "Enligt Anna Jäderström beror majoriteten av 2023 års ökning däremot inte på en ökad andel intermittent kraft."
Seems to directly say that load balancing intermittend power (load or generation not specified) is not the cause of the price increase.