I've had nested tmux sessions before. Usually I'll have a couple tabs on my local device, and then may want to ssh to a remote server and interact with a persistent tmux session there.

I usually make a point of setting different prefix keys in that case though.

I often run tmux in ssh in tmux. The outer tmux is just for my local terminals. The inner tmux is mostly so I can reattach later. Though it is also nice to easily get two shells on the remote machine side by side.

Maybe they mean tmux is not installed on the remote machine but screen is, so screen is inside the ssh session, inside the tmux session

Maybe for avoiding shortcut clashing.