That gets you scrolling, yes, but not in the same way as without tmux.
Without tmux, your terminal emulator uses its scroll-back buffer to render as you scroll, not requiring any intermediate copies of the lines of text scrolling into view.
With tmux, your terminal emulator no longer handles that, and instead tmux must pass a copy of each line freshly scrolled into view to the terminal emulator, which involves an intermediate copy of that text and re-triggers the terminal emulator’s parsing (as far as your terminal emulator is concerned, it has never seen this line of text yet). All of that (and some other subtle overhead I’m not mentioning here) are completely avoided outside of tmux (or more specifically: whenever the terminal emulator is free to manage its own scroll-back).
Without tmux, your terminal emulator uses its scroll-back buffer to render as you scroll, not requiring any intermediate copies of the lines of text scrolling into view.
With tmux, your terminal emulator no longer handles that, and instead tmux must pass a copy of each line freshly scrolled into view to the terminal emulator, which involves an intermediate copy of that text and re-triggers the terminal emulator’s parsing (as far as your terminal emulator is concerned, it has never seen this line of text yet). All of that (and some other subtle overhead I’m not mentioning here) are completely avoided outside of tmux (or more specifically: whenever the terminal emulator is free to manage its own scroll-back).