Generally when discussing impostor syndrome, one of the major fears is of being "found out", and in the case of engineering, "fired in disgrace and drummed out of the field, never to have a job in it again". That's not narcissism. It's a legitimate fear. It may not be a fear grounded in reality, but it's a legitimate fear.

I would argue that the best engineers focus on themselves quite a bit--they just focus on improving themselves and their strengths and weaknesses instead of their relative ranking among others.

There's a world of difference between introspection and narcissism.

Being self-absorbed is a vice, but being aware of your limits and how best to push them is a virtue.

While that may be true for some, a number of highly effective people in various fields seem to have a sense of dissatisfaction that pushes them to do more. That sense can often apply to their own self-assessment.

I don't think anyone's immune to thinking about how they rate, but the best know they're good, so it's usually just a passing thought they spend very little time on.

This is actually pretty interesting. Maybe imposter syndrome is an inevitable side effect of our aversion to credentialism.