It's handy to know sometimes, but if I have a sane window manager at hand then I don't use tmux. They're not common, sane window managers, so I use it occasionally.
Of course you can:
1. create tmux session, create splits/panes 2. detach 3. asciinema rec 4. attach (inside asciinema rec)
This way asciinema records your whole terminal, and tmux running inside of it.
In other words, if you have it like this:
terminal -> tmux -> pane -> asciinema rec
asciinema can't see outside of the pane its running in. However, if you have it like this:
terminal -> asciinema rec -> tmux attach
asciinema sees all of it because its parent is your terminal, not tmux pane.
StumpWM similar concept but more powerful.