It is not optional if you have a backup password set. Once you log into your account using your phone number, you still need to enter your backup password once

It is not optional if you have a backup password set. Once you log into your account using your phone number, you still need to enter your backup password once to decrypt and actually start generating codes. The reminder you are talking about is when you are already using Authy to re-enter your backup password so that you dont forget it. That is the prompt you can dismiss, not the once needed to setup Authy on new machine.