They may well have not rode up with the killer. Get a set of fire keys and you can force an elevator to go to any floor, regardless of what fancy keys or tokens the elevator uses to restrict access. And since the elevator opens directly into the apartment, you don't even have to get the victim to unlock their door.

Edit: I can buy the FDNY fire key on Amazon. Anyone could have pulled this off.

Edit 2: The NYT article (https://www.nytimes.com/2020/07/14/nyregion/dismembered-body...) does say he was in the elevator with his killer. It seems the ABC7 article is lacking a lot of information.