False humility? Countersignalling? Depression? I don't want to make an internet diagnosis or mind reading, but from my view these options seem more likely than the hypothesis of low intelligence.

From the context I ruled out countersignalling and for what it is worth my impression was that the humility was real, not false. Given that I err on the side of cynical regarding hypocrisy and had found some of XiXiDu's comments disruptive I give my positive evaluation of Xi's sincerity some weight.

I agree that the hypothesis of low intelligence is implausible despite the testimony. Addition possible factors I considered:

• Specific weakness in intelligence (eg. ADHD, dyslexia or something less common) that produced low self esteem in intelligence despite overall respectable g.
• Perfectionistic or obsessive tendencies which would lead to harsh self judgements relative to an unrealistic ideal. (Potentially similar to the kind of tendencies which would cause the idealism failure mode described in the opening post.)
• Not realising just how stupid 'average' is. (This is a common error. This wasn't the first time I've called 'bullshit' on claims to be below average IQ. Associating with highly educated nerds really biases the sample.)

(Unless the context was something like "intelligence lower than extremely high"; i.e. something like "I have IQ 130, but compared with people with IQ 160 I feel stupid".)

That would have been more accurate, but no, the context ruled that out.

I'm curious whether XiXiDu's confidence/objective self evaluation has changed over the intervening years. I hope it has.