help messages of aurora job kill says
If INSTANCES is omitted, then all instances will be operated on.
However, when the instance is omitted in the command, the command reports
The instances list cannot be omitted in a kill command!; use killall to kill all instances
IIRC the /instance was dropped here in favor of aurora job killall because some people accidentally killed all instances of some very large production services rather than only a single instance.
The command needs a new help message to prevent confusion.