Speaking from experience as an introvert who suffered social anxiety...
A lot of people (probably extroverts) don't respond well when you're expressing anxiety and/or doubt. Clients won't want to work with you. People won't want to be around you at parties. Co-workers will speak behind your back about the weird person. Etc.
Especially with other neurospicy folks, it's a very easy way to bond. My issues might not be the exact same, but it's close enough that they can empathize and grow closer as friends/coworkers.
I'd rather own my mistakes than be a walking contradiction.
To be fair he's probably overconfident. But picking up any sense of false confidence(not a shy buddy attemping to 'fake it til you make it', more serious matters) is a massive red flag to me that I can't trust their judgement of their own knowledge or capabilities. Someone who says straight up it will be difficult gets a lot more trust from me.