I hate to be that guy, but why would I use this over tmux? All of the features and qualities noted in the readme do not feel novel or unique.
hrdx looks great, but as it does it's own multiplexing, I'm not sure the post's rationale is suggesting something like hrdx.
But fair question. If tmux already fits your workflow, there may be no reason to switch. hrdx is more opinionated around running multiple coding agents across projects: it provides a workspace and pane sidebar, agent activity and completion status, built-in launch/resume flows, persistent sessions and layouts, and an external control API. The tradeoff is that it is much younger and less configurable than tmux.
Try it, play around with it, love it or hate it :-)