That is true. It seems somewhat beside your original point, though, unless you are arguing that that difference cuts it clear which terminal emulator should handle scrollback

No, my point wasn't strictly about scrolling. My line of reasoning is:

1 tmux is needed to handle multiplexing

2 tmux also handles scrolling

3 if you need tmux for other things already, you don't need scrolling in the terminal emulator

I wouldn't say that, by this reasoning, it is the right place to implement scrolling, just that, since it's a solved problem, re-implementing it would be a useless duplication of effort.

But in this specific case I'd concede that the way tmux scrolls is not at all comparable with how a X terminal emulator would be expected to scroll.