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).

tmux does support scrolling and copy-paste, but they often work subtly differently than the way they do in a native terminal. For example, when I try putting `set -g mouse on` in `~/.tmux.conf`, scrolling mostly works fine but if I scroll up and then start typing or press up, I won't jump right to the bottom of the session the way I'm used to in native Alacritty (which I will often do as a way to return after scrolling back through a bunch of print output). This isn't a huge deal, and you can definitely get used to it, but it can be frustrating if you like how your normal terminal emulator works and don't want it to change.