They may have gone over the top with their presumptions about financial situations, or how you dress and so forth, but I WILL say they do have a point in general to a certain extent. Being on numerous message boards throughout the years, I've noticed a direct correlation between how people type, and their intelligence level. It's one thing to make an error here and there. We all do. But when you make the same mistake repeatedly (can't tell the difference between there they're and their, or use an apostrophe before the s to indicate a plural form of a word, etc), it's no longer "an honest mistake". It shows that you are ignorant about proper grammar. Otherwise, to claim that frequent errors are "an honest mistake" indicates that you are too lazy to proofread a few words. Neither one of those reflects well on a person's character or intelligence.
Again, this may not pertain to you, as I don't know you much from here, much less the forum in question. I'm just saying in general, I can see how when people say "I don't care about internet grammar", it can come off as sounding like someone is lazy in general. If hitting the punctuation or shift keys, which are centimeters away from your fingers, is "not worth it", I can understand the presumptions that there's probably a whole lot more that's "not worth it" in that person's life.
You can't treat other things in life like a text message.
