They used a loophole: to adopt Euro you need to be in ERM-II for a while as a pre-requisite, and Sweden found out that there is no rule imposing a timeframe for it to join ERM-II so it is deliberately not joining. IMO this is complying with the letter of the agreement it has with the EU but not the spirit.

As far as I understand, the EU closed this loophole for new members but it is turning a blind eye for Sweden.

It is fascinating to me that in practice there is this reversal of roles: Sweden is supposed to adopt the Euro and yet has a free-floating currency, and Denmark that has an opt-out deliberately chose to be in ERM-II and has a currency that is pegged to the Euro.