I mean, cool, but he's 64 and is retiring. He started at the UN in '92. He announced that he would retire in March of this year. Most sources list that now.
So it's more of a 'i don't need to deal with this insane bullshit so I'm taking my very healthy retirement package and heading out' more than a 'im taking a moral stand and leaving my dream career over this insane bullshit'
More often, threat of resignation is a political tool to sway others. I mean, if everyone knows that the person threatening to resign is that good and irreplaceable, others would cave in. I don't know how the UN works behind closed doors, and how capable Mokhiber is, but we'll see if this works and the UN comes begging for him to return.