I gather most of them are, but you're missing part of the sentence: he's been sentenced to "forvaring", aka "indeterminate penalty". This means a baseline 21 years (as 21 years is the maximum determinate penalty outside of crimes against humanity) but it can be extended indefinitely, 5 years at a time.
It's not a penalty though. People who are severely mentally ill wouldn't just be allowed to wander about in society either. If someone poses a thread to them selves or others, then they would be kept under guard. But that is not to punish them.
How is this better than a life sentence? I think it's kind of disingenuous to pass this off as a "Well they're staying true to their morals!" if the end effect is that this guy spends his entire life in prison, with no chance of getting out in 20-30 years.
Those sentenced under forvaring can apply for parole after 10 years, and after 21 years the case is reviewed every 5 years (to decide whether the inmate is still dangerous to society) and parole can be applied for every year.
There is very much a chance of getting out in the timeframe, if the inmate reforms.
I gather most of them are, but you're missing part of the sentence: he's been sentenced to "forvaring", aka "indeterminate penalty". This means a baseline 21 years (as 21 years is the maximum determinate penalty outside of crimes against humanity) but it can be extended indefinitely, 5 years at a time.