Being Indian (so also kind of black) I'm not sure if the allaged insult is enough to warrant walking away from SOO. My thoughts are that if anybody is racist to me at work that I won't be harrassed or bullied out of a job or any other lucrative opportunity. I'd just find another way to deal with the problem. So I would react, but not in a way that diminished my own welfare or opportunities.
But despite the odd flare up here and there actual face-to-face racism is pretty rare in Australia. Its more the stuff you don't see. At any rate, Johns and Tahu have known each other for about a decade. If Tahu has heard this stuff before then why not just tell Johns to shut it and as Fathead said, why haven't they had a major blowout earlier.
And though there's been a few incidents I don't think league has a racist culture. This is a sport that has had long ties to the Indigenous and Pacific Islander communities. That said, I wouldn't underestimate the stupidity, as opposed to malice, of the Johns brothers.