I figured it out. Either I copied something wrong or the original tmux + nixvim fix I used didn’t work, but here is the fix that ended up working, from this post:
{
plugin = resurrect;
extraConfig = ''
set -g @resurrect-processes 'ssh "~nvim"'
set -g @resurrect-strategy-nvim 'session'
resurrect_dir=~/.local/share/tmux/resurrect
set -g @resurrect-dir $resurrect_dir
set -g @resurrect-hook-post-save-all "sed -i 's| --cmd .*-vim-pack-dir||g; s|/etc/profiles/per-user/$USER/bin/||g; s|/nix/store/.*/bin/||g' $(readlink -f $resurrect_dir/last)"
'';
}