I find the error message a bit misleading as it fails to explain that in this situation the @username isn't taken into account in the character count (since @username won't be displayed in the posted comment, as username is the poster):
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Yes. The system lacks consistency and clarity when it comes to this point.
Whether you agree to the current explanation of Jeff why the @username gets stripped out or not, we still should make the system easy to understand to users. Who didn't count 10 times when he/she first saw this error message? At least I did.
So yes, the text should make the reason behind it clear. Another thing, not very related, I can think of is this: shouldn't it always strip out the @username when counting the characters? This will make the behavior easier to understand in all circumstances. Maybe I will do a feature request on this.