Experience doesn't get to dictate racism.
On the other hand, he's black and he's dressing different from other people at the office, so it's "thuggish"
The power differential between them. This guy's racism exists on tumblr and is targeted at noone, the racism he suffered at work was going from a manager to a lower employee. It's the same reason teacher-pupil relationships are never acceptable, no matter the age or maturity of the student.
I wish that neither Kixeye employees nor the blogger made racist comments, but we can decry one as worse than the other.
While the author certainly doesn't deserve to be treated with disrespect, I think there is some looking inward that needs to happen here too.
There's another definition, which is talk that's designed to maintain & reinforce the institutionalised power structure among races. Right now, if modern USA life was a video game, "white male" would be an easier difficulty level than "black male". There are statistically less problems for the "white male" group. Racist talk is talk that re-enforced that imbalance. This definition is harder for some people to accept because it means that you need to look at yourself and think about what power imbalances you might be benefiting from, and it means you can't just do s/white/black/g and make it just as racist.
So no, it's more racist to say "dumbblack" than "dumbwhite"
Also, that judgement really doesn't affect the manager, as he was just able to walk out of that meeting and continue to be a shithead.
It is racist. Anti-white racism is a common feature of modern black culture, but it's not considered controversial because of the traditional oppression of black people and the lingering effects that has today.