C-b is pretty bad for the same reason for users like me with with well-worn emacs key memory. I end up using the backtick, but it does make sharing a session that little bit more difficult.

adding

  bind C-a send-prefix  

to your .tmux.conf file will allow you to type C-a twice and still get to the front of the line

I'm not sure about tmux, but you can still move to the beginning of the line by pressing C-a a in screen.

Actually, it is not just urban legend but part of their FAQ ;-)

https://github.com/tmux/tmux/wiki/FAQ#why-is-c-b-the-prefix-...

Nevertheless, I think they should have changed that in the release version and configure their dev environment. After all, it is pretty uncommon to run tmux inside screen for most use-cases.