It is not a back up because the secret key (activation key) is only needed once when you activate 2FA and also only once to deactivate 2FA.
It is right but only if your phone, tablet is not dead and you can get 6-digit code from your 2FA application.
What will you have if your phone is dead? How will you deactivate the 2FA of your account?
It is when you need the secret key (activation key) to install that 2FA account to another phone or tablet and get 6-digit code to log in or to remove your 2FA.
The secret key (activation key) is not a back up as I said because you don't have a copy of it and only the platform have it that's why you have to make a copy of it to create a back up of the key.
Are you sure?
It is the only way you can backup your 2FA account and won't have to rely on exchange support.
What you always need is the Authenticator to copy the code which is needed before you can log in.
It is the code to login your exchange account or to remove 2FA on that account.
How to use Binance authenticator for 2FA. You will have two options when activate your 2FA, QR code or 16-digit key and it's your backup too.
You can scan that QR code or type that 16-digit key on another device to get a same 2FA account to use.