Quite the hyperbole. The author even points out how this was done without realizing it — purposefully misspelling certain words to get the right character counts.
Chapter 2: 18 justified lines. "missles" for "missiles" but is consistent throughout.
Chapter 3: 61 justified lines. "missles" consistent again. "limitid" for "limited" (same length). "powerfull"/"usefull", yes, but again looks to be consistent throughout. "liek" for "like" (same length). "essentail" for "essential" (same length).
Chapter 4: 88 justified lines. "missles" and double "l" consistent. "mroph" for "morph" (same length). "eleminate" for "eliminate" (same length). "insure" for "ensure" (same length). "wreckless" for "reckless" but in a non-justified line.
Ought to be possible to test this hypothesis (is the justification done by conditional misspellings?) with some simple scripting.
edit: however, it appears that's not what the author did.
A couple of missed words or odd linguistic choices, but compared to the number of lines, not many.
The one specific misspelling that the tweeter points out is consistent throughout.
I think you're right, the misspellings are independent. For a few words (like "begining") the text contains the correct and a misspelt word. But for most misspelt words ("carefull" for example) the text contains only the misspelling.
Thus, the text being fully justified is independent of the misspelling.
The document being an in depth write up of Super Metroid probably also says something about the person's character!