I find it rude when people focus on spelling rather than the content of a post. That happens all the time though. It’s like people don’t care what the person has to say if it’s not spelled perfectly.
People who are dyslexic can’t help spelling wrong. Correcting them will not help them write the words correctly the next time and they don’t need to be constantly reminded of how bad their spelling is. Being constantly corrected by others can get very annoying and frustrating and it can have negative effects on the person’s self-esteem. Nobody likes to have their mistakes or shortcomings pointed out all the time and if people always comment on the spelling rather than what the person has to say, it gives the impression that people don’t take them seriously or don’t care about what they said, just because of the spelling.
So you would not have done this person any favors by correcting his post. You could have just made him feel worse about it. I think the reason why he mentioned his spelling problem in the post was to indicate that he is well aware of his bad spelling and can’t help it and would rather be left alone about it.
There are some dyslexics who don’t mind having their spelling fixed by others, but there are much more dyslexics who wish people would just not care so much about the spelling and focus on the content for a change.
Spell checks are not always useful for dyslexics either. They don’t correct everything. There are some words they don’t recognize and if the word is really badly misspelled they don’t always recognize which word it was supposed to be. They don’t understand context, so they don’t correct misspellings if the misspelled word looks like another real word (for example they don’t usually recognize the difference between “we’re” and “were” or "has" and "had"). Spell checks that ask you to confirm which word you meant to write (such as the spell check here on Y!A) can be a problem too, because a dyslexic won’t necessarily recognize the correctly spelled word from the list. It also takes extra time and effort to go over the corrections like this.
Spell checks can be rather time consuming. The time they take may seem insignificant when you’re writing one post on a forum, but imagine the time it takes to have literally everything you write spell checked (every post you write on forums, every e-mail, every instant message etc.). On top of that, dyslexics often read and/or write slowly, so in total they must spend much more time than others when writing. Sometimes there just isn’t much extra time or patience for spell checks.