Sorry, that's sophistry. "Deals IN" cannot mean "deals WITH". The scene should be understood exactly as it plays. A "Sith"-like attitude insists on an innate "goodness" and "badness" (or more appropriately "us-ness" and "them-ness") in everything. The "Jedi" understand that no object or action is intrinsically good or evil. Intention and effect matter, turning black and white into shades of gray. The fundamental difference between the sides is not "good" and "evil". (Everyone has the potential for both.) The real difference is between a desire to control one's environment for personal power and security, and the desire to serve the greater good of all. A "Sith" believes in him/herself, forming temporary alliances only for the purpose of gaining greater personal advantage. A "Jedi" seeks and fosters the good in everyone, risking and sacrificing personal safety and reputation to benefit the whole. Their actions may occasionally resemble each other but their motivations are very different.
The "Jedi" mentality DOES have "absolutes" of a sort. The preservation of life (particularly intelligent life), freedom (physical and intellectual), justice (without regard to status) and the like are enduring values for "Jedi"-like people. But they understand that the universe is morally neutral, that often one value is pitted against another by the unscrupulous, and that sometimes the best that can be done is a moral compromise.
The "Sith" prefer to concretize their "morality". Disloyalty to THIS leader is wrong. Partaking in THAT specific activity is evil. An uncensored discussion of social alternatives is morally decadent. Categorically, without exception, never mind the intent. A "virtuous" act is always "good". A troublemaker is always "bad".
A "Jedi" can observe the ethical situation, note changes, and change his mind about a good idea implemented badly, or a disreputable strategy that actually produces good. A "Jedi" can apologize for being wrong. A "Sith" cannot afford to be inconsistent for fear of losing credibility. Any change in allegiance must be framed and propagandized to appear as part of the plan all along. The "Sith" is about appearances rather than reality. The world is never black and white, but the "Sith" pretends it is. That is what is meant by "dealing in absolutes".