insideflyer just quotes airline press releases. If they add own comments, it will always be in the negative direction. Their tone towards SK has improved significantly since they lost norwegian as main sponsor. 6 months ago they would have written something like despite SK renewing their fleet they will still for years have an old fleet compared to other airlines.

That said. SK is renewing it's fleet and as stated in the article they have ordered a high number of new aircrafts compared to their fleet size, and everytime they receive a new A320 on of the old B737/A32x will be scrapped. The same goes for the A340's. They will be scrapped as the A350's arrive. But it will be +5 years before the current order is fully fulfilled and at that point in time, they will still have a fleet with an average age of 10 years.
Also please note that the number of aircrafts on order in the insideflyer article includes aircrafts delivered within the last 3-4 years.