The problem is that, from the inside, imposter syndrome feels like an accurate, negative self assessment. Writing off the latter as the former is not the right way to go; for every person who's wrong in that direction, there's probably another who's right in the same direction.
What we should be doing is teaching people how to make well-calibrated self-assessments, not repeat the ridiculous line that "lol no one knows what they're doing, everything's guesswork". We should teach people how spot the evidence of success that they miss -- but also the evidence of failure.
The phenomenon is highly common among high-achievers which is why Popova references it.