Looking at the deal it's an acquisition offer, meaning it comes as part of a new service, where as you're an existing customer looking for a new deal, new customers get welcome deals, you would of already had yours when you joined.
To get that offer you would need to order it, get a PAC for the original sim, move to another network, then move from that new network back to BT or you could just ring the loyalty team and see what offers they have, you will not get the reward card though.
Click through the offer and see where it takes you.
It doesn't say anywhere that i can see that it's only open to new customers
To me that's false advertising
It's not just for new customers, existing customers taking that service for the first time can have it, so if you want a 2nd sim you qualify, you're talking that you want to change an existing service, which isn't what the offer is for.
Have you even tried to see what deals you get ? login to www.bt.com/mybt and manage your package, I bagged myself a 6GB plan for £7 recently (Includes £5 BB discount) but there were other really appealing deals to move up or down to.