Yeah that's a good point, "lightweight" was probably not the best term. I meant lightweight in terms of the cognitive overhead of picking up a new tool, not really in terms of performance. I would expect that shpool will perform better on some workloads and tmux will perform better on others (though in both cases they are probably good enough).
The main difference is that tmux is a power user tool that you generally invest in configuring and tweaking to get just right, while shpool is meant to be set up quickly to solve the specific problem of dropped connections and then mostly forgotten about.