shpool doesn't allow splitting screens either! But I guess in my head, screen is largely for resuming a shell connection on a remote host while tmux is a window splitter.
I recently finally switched from screen to tmux and I found that it's pretty seamless (especially because I use vi keybinds in tmux still). Some machines at work don't have screen anymore.
Incidentally, this "screen absent" is why you should always volunteer to help create base shells (programmatically via packer not by hand) - emacs and tmux are always there when I made the image.
I don't make the image.