Based on the prior cited: Can. 983 §1, Can. 984 §1. & §2., it is correct that the seal of confession cannot be broken on any circumstance including confession of murder.
However, there is an additional stipulation concerning murder, Can. 977:
Can. 977 The absolution of an accomplice in a sin against the sixth commandment of the Decalogue [Thou Shall Not Kill] is invalid except in danger of death.
This essentially means that absolution cannot be given for murder, so while the confession seal is upheld no absolution is given meaning practically speaking the sin is unforgiven. With the exception of a deathbed confession.
So the more proper answer would be that yes the seal of confession cannot be broken under any circumstance, however the confession of murder is not considered licit and will not be absolved either.
I apologize I do have the proper citations for this in canon law for case precedent in doing this, but based on my own observation & hearing the accounts of multiple confessors. How this usually works out in practice is the confessor will inform the person that for their penance they must provide their confession to the lawful authority and face secular justice, and the serving of their time in prison or whatever the lawful punishment is is considered an act of their penance. Therefore satisfying the need for a just punishment while also absolving the penitent.
Unrelated to the question, but for those curious there are additional categories of sin where the priest is not permitted to grant absolution without dispensation from his bishop. One related example I'm aware of is abortion, where while normally this would be covered under the 6th, Francis has granted dispensation for priests to absolve this one. I'm uncertain if their is a canon law citation to provide, but just a recent event I'm aware of, happy to clarify further if needed.