There is a rule to avoid freezing the main chat room of a site, but it doesn't identify any particular room as the main room. Rather, the last remaining unfrozen room of a site will not be frozen.
The Duga room is considered an active room since a normal user talks there fairly often. The normal user is actually a bot, but that's an external bot, so Stack Exchange doesn't know that it isn't a human and considers the room to be active. Our main chatroom has posts from RSS feeds, but those are identified as automatic and don't count towards freeze-preventing activity.
Does anyone pay attention to the activity from Duga? If not maybe it should be shut down. If people do pay attention, maybe it should be in the main room. Or the bot could be reengineered to provide a RSS feed instead of directly posting into the room, then the Duga room would have been frozen (which doesn't prevent the RSS feed from posting).