Pretending that both sides are equal is not always helpful. Bad actors do exist, and pretending that everything is equally indeterminate is a fatally nihilistic and dangerous view. Sometimes we actually can tell when something is a lie, and we should treat it appropriately. For a tool that retrieves genuine results matching a query, that means down-ranking them.
For the purposes of current event queries, disinformation is certainly spam. The purpose of spam is to fool you into doing something you probably wouldn't otherwise do, usually based on false pretenses. Disinformation is the same. The action it promotes is more indirect - it seeks to influence beliefs, and therefore voting patterns and soft power - but it is still spread with a purpose that is disingenuous. It disguises it's true intent and is dangerous to the user. It is effectively spam applied to the domain of politics and current events, and should be treated as spam is. It should not be a valid result in an information retrieval service.
I agree that this ranking could be the start of a slippery slope. But DDG is being transparent, and you can still get to RT if you disagree. This is a good measure for people unfamiliar with Russia news sources and propaganda networks, who are the people most likely to be susceptible to Russian propaganda.