One small addition: you can do all the work on your own if you have to.
But you also know how to delegate it to a team to get it finished more quickly. And how to keep the team on track so technical debt is minimised, while reliability and user usefulness are optimised.
Really, there are two definitions and two different skill sets.
One is "competent experienced professional developer". The other is "competent manager of other developers."
Of course, now we have to define "complex", but the salient part is being able to work independently.