Manslaughter is treated differently in every jurisdiction in the US, but the level of Manslaughter that the offender was changed and convicted with is legally adjacent to a murder charge. It’s just that the prosecutor didn’t believe that they could prove - beyond a reasonable doubt - that the offender actually intended to kill the victim rather than, say, frightening them or, perhaps, injuring them. So they went for a charge that they knew they could prove in court.
Colloquially speaking, this person murdered the other, no doubt. But what could be proven in court, given the evidence, was a degree of manslaughter.