The ability to allow contributors to conttibute code is a bonus of it being open source but even if it doesn't accept or merge pull requests, it's just as well imo.
Too many projects just dump the code onto Github with no attempt to create a community around it. Community participation is ultimately what makes or breaks an open source project. I'm not saying one has an obligation to the world to service pull requests, but if you release something and make zero effort to engage, then don't be surprised if your project dies.
In this case, the codebase is also sufficiently complex/arcane that it's hard for anyone to just pick it up and evolve it meaningfully without any aid from the original author. Without knowing all the details of Hyperdex's internals, I do think that it might be a worthwhile thing for someone with sufficient time and interest to resurrect it, though.
You need to be an invested person to casually read the sourcecode of postgres or random database project.
I have some ideas on HTTPS://GitHub.com/samsquire/ideas4 of how I would cause programming to be easier to pick up and run with a piece of mature software. There is inevitably A LOT to learn to onboard yourself with a large codebase, it takes months or years to get familiar at work so it would take the same or longer for open source.