Confused - why are they sending me a new SIM while I'm trying to activate an existing SIM?

I'm in possession of a spare ('Bonus') Smarty SIM, and mine does not say to activate it through the app - it tells you to go to smarty.co.uk/activate.

The activate page asks you to log in if you already have an account. As a new user, it is not expecting you to have an existing account so it's not expecting you to log in. Instead, it wants you to remain logged out and enter the SIM number in the box below that. It will then get you to pick a plan and create an account as part of the process of activating that SIM.

Many people make the same mistake you have and end up ordering a SIM through the mail. I did start to wonder if Smarty's site is actually broken and it's not actually possible to activate a bonus SIM without ordering one to come in the post, but I've recently been able to test this out and it's not the case - if you stay logged out and follow the activation process through, it is absolutely possible to choose a plan for and activate a SIM already in your possession.