We aren't talking about short couch potatoes being scared to apply to the NBA. That's a justified sense of inadequacy.
We're talking about people switching from one fairly normal job to another one that probably has a lot of overlap with what a person was already doing. Imposter syndrome is the unjustified sense of inadequacy.
Its not a binary thing though. Some people would have a little bit of doubt transitioning to a slightly different role and that's normal. Change is scary to lots of people to varying degrees. However, some people are very competent, yet might be afraid to apply for a higher level position or a slightly different role because they fear that they are actually terrible and that they've gotten where they are due to luck and graft. That's imposter syndrome.