Yes, anyone can join. If their wording is really "complimentary", that is indeed confusing. But if their wording is slight different, and you rephrased it as "complimentary" what they could really mean is that you're automatically singed for Avis Preferred with that Amex Platinum card already linked.

Anyone who signs up for Avis Preferrred has to specify a credit card to link to their Avis Preferred account, and that credit card is automatically used on all rentals done under that Avis Preferred account.

So I wonder if what they did was automatically enroll you in Avis Preferred and pre-set your linked card to be your Amex Platinum card? (But if so, it would seem confusing if they didn't explain that!)