It's absolutely their prerogative to give him a guarantee.
Or, they can choose to undermine their credibility as a champion of human rights by providing no guarantees of humane treatment to any person under extradition.
Many countries opt to follow this route (not nice countries, mind) and thus voluntarily "fail" to extradite a great number of guilty people as a result. Refusal to respect human rights often comes at a cost.
Sweden's hands were not tied on this issue and it was a reasonable request. The reasonableness and the apparent politically motivated intransigence of the Swedish justice system (something noted by eagle eyed GCHQ employees in leaked emails) is a large part of why the application for asylum was approved.
This is also probably why he made the offer ~30 times.