He wanted to type “bigger”, mistyped it as “nigger” (as “n” is right to ”b”) and autocorrect didn't apply. At least not on iOS, which he uses.
Hower, iOS autocorrect will offer “nigger” in certain cases, as when you are typing “migher”. (This could be a side effect of me entering “nigger” as a shortcut for “bigger”, to see if it's possible to block the word altogether. didn't work.)
Edit: Made a mistake. I keep mixing up the order in which to enter the shortcut and substitution, so I actually made “bigger” a shortcut for “bigger”. Whew. That could have been awkward, if I tweeted about something bigger.
Anyway, when I make them in the correct order, “nigger” will get autocorrected to “bigger” and “migher” won't get autocorrected at all.
So there's a pro tip: Enter all offensive words as abbreviations for something you probably wanted to type.
Thankfully, not much of of a problem for Germans, as “einweniggroesseralseinanderesding” is unlikely to mistyped as “personmitdunklerhautfarbeundschwarzenhaaren”.