On any post with a lot of comments, I see obvious misspellings, not just loose for lose, but things spellcheckers catch easily, such as descretely for discretely, etc.
This surprises me because commenters here likely have higher IQs than most other public internet forums.
That should only surprise you if you assume higher IQs correlate to better spelling. Have you seen studies that indicate a correlation exists?
Assuming for the moment all people who win spelling bees have high IQs that doesn't mean all people with high IQs spell well. That's just inappropriate generalization[1] on your part.
I think you're also failing to consider people leaving comments on forums are often more concerned with the content getting their point across than making errors (e.g. you wrote selling bees instead of spelling bees).