OED:
nigga
1925, also niggah, representing southern U.S. pronunciation of nigger (q.v.).

Dictionary of American Slang :
slang
n.
nigger; Negro. (Streets. Nicca is pronunciation and/or spelling variant. Restricted to black to black use. White to black use is provocative.)
nigga

WordNet® 3.0, © 2006 by Princeton University:
noun
(ethnic slur) extremely offensive name for a Black person; "only a Black can call another Black a nigga"


It most certainly does qualify as an offensive term. That being said, the racism control shouldn't be black and white like you are making it to be. The point of the no racism as a rule is to keep people being unnecessarily racist. This clearly wasn't even meant as an offensive term directed at anyone. If you are that touchy about "racism" you need to get the fuck off the internet. You've taken the letter of the law and disregarded the spirit of the law.