alt Hacker NewsPublisher is the equivalent of InDesign. It was meant for brochures and so on. If you want to write a long technical manual today most people use Word. In that respect we are using less powerful software today than our grandparents.
Note: Adobe bought FrameMaker and continues to sell FrameMaker. But Word has captured the market not because of its technical merit but because of bundling.
I have never written any technical manuals, but I'm surprised that Word is the choice of tool. How does one embed e.g. code easily in the document? I feel there must be a better way to do it, maybe some kind of markdown syntax? Latex?