Picking up the car todsx everything worked fine. I didn't sign any documents that say that I need to be a member of anything. Also that link doesn't say anything about it.
Signed the contract which didn't mention the discount and wasn't asked about anything. So I think it's fine. The line about discounts/bonus was empty in the contract.

It would be hard to argue that links from sixt that you can find directly via Google when searching sixt and discount linking to a page that doesn't say you cannot use it or needs you to be a member causes any problems. There was nowhere in any link to the terms a mention of corporate discounts/membership or similar.