What is it that makes you think Reddit communities are a different animal? Were Reddit to disappear, I'm sure those communities would find new homes. From where I'm sitting, the problem with basing Reddit's valuation on those communities is that Reddit doesn't own those communities.
You can't always tell a community what to do (like how one can't build an HN replacement and expect the community to simply follow you there). If the steps Reddit takes to try to make a big return on investment anger those communities, the communities are free to leave. My view is that the network effects of Reddit are not nearly as strong as those of a Facebook or Instagram, and even those appear to be having some difficulty getting as much value out of their communities as they want.
