No, murder has a mandatory life sentence. Sicherungsverwahrung is totally distinct from that, as is the declaration of „special gravity of guilt“ that the court can impose.
Everything less than life is capped at 15 years (manslaughter etc.)
Life sentences mean exactly that: until death. But for constitutional reasons every convict must have hope that he can be released early. That‘s what those regular reviews (after 15 years or more) are for.
Many convicts get released early, because they sat in prison for a long time and pose no threat anymore. Others are released for medical/humanitarian reasons.
But this opinion that murderers always get free after 15 or 17 years is pervasive and utterly wrong. And it leads to people demanding excessive punishments, because they feel they must demand much more to have evil people do time.