I work close enough to systems engineering to have some insight: The good systems engineers are strong generalists, have one core competency with deep knowledge and expertice, good enough knowledge in related fields to understand and work with experts, and are generally good at engineering.
Complex job, but a fun one! And crucially important for any product more complex than a dish washer.
But please, don't steal the name for some software "engineering" role...
I think parent is refering to the fact that in a formal context (think large Aerospace systems), a Systems Engineer is the role that oversizes all interfaces mech/electric/software and makes sure there are no requirements misalignment.
In that sense the naming has been misappropriated in the SV tech circle to mean a "backend engineer that actually understands what is going on under the hood" sort of role.
Complex job, but a fun one! And crucially important for any product more complex than a dish washer.
But please, don't steal the name for some software "engineering" role...