As someone with no credentials or official training, and someone who also works at a big megacorporation as an engineer, imposter syndrome hits me fairly hard, especially during interviews. It's really hard to compete with the people who actually have proof that they know what they're doing, and I'm usually interviewing for a job that has a strict requirement of a "bachelors in CS, Math or Physics, Masters preferred"…
And yet... as an independent consultant, I'm often brought in to projects worked on by these folks who "have proof that they know what they're doing". If they knew what they were doing, almost by definition, they wouldn't be calling in an outside consultant to fix broken shit.
There are often many reasons - cutting corners due to time or budget, or inheriting something that was bad and not having support to fix it, or not knowing how to fix it. But often if I'm able to talk to these folks... they didn't know what they were doing (and often didn't know they didn't know), so the result was a mess.
The "1% dude" with his CS degree probably has never had to acknowledge to a boss that they made a mistake, or are in over their head, or... haven't had to deal with their own code 5 years later and clean up their own mistakes, let alone others'.