Would you consider dyslexia an excuse, or English as a second language? Or even semi-illiteracy? I think it's definitely possible to be very clever and still not a good speller, and not bother to double-check on an Internet forum.
Einstein was notorious as a poor speller, which suggests he didn't double-check himself.
Ignorance and IQ are certainly different. But IQ is generally measuring "problem solving ability", and when to use a contraction is a problem. If one is very clever, one tends to have that particular problem solved. But, whether the misuse is from inability to solve the problem, or a choice to not solve the problem ("can't be bothered"), either of those is a reasonable predictor of job performance, health, and various other life outcomes.
On "Einstein was notorious as a poor speller" – after moving to the US, Einstein became completely bilingual but could never recall how to spell words correctly in both German and English. This is not the same issue as when to use a contraction, and isn't the same as "not bothering".
The ESL student learns, understands the theory of, and tends to be careful with contractions. ESL errors look and sound quite different than most of these errors encountered online.
The illiterate "I'm too cool for your TL;DR grammar-nazi wall-of-text" types have failed to prime they're Baysian neural networks by reading or doing homework, and generally have no idea their "doing it wrong".
I see what you did there.
As far as using common homonyms and misspellings as predictors of "job performance, health, and various other life outcomes", well, I'm not quite sure that's reasonable. For example, xd's comment below:
http://news.ycombinator.com/item?id=2868142
has an error in it "Too many people will spill there emotions" but I don't think it's reasonable to predict too much based on that. There's several reasons these errors are common, not just inability or indolence.
There's often a slip between mind and the keyboard - I know I often simply type the wrong thing even when I know full well what the correct thing is. Muscle memory or something.
Edit: 'their "doing it wrong"'. Ah yes, very good :)