Yes you need a Bank Guarantee!
At least it was mandatory in 2007, when I applied for the Platinum IDC, they asked my bank to guarantee the card for an amount of 75k Usd dollars! I know it's a huge guarantee! but my bank accepts the offer! and then a card was sending out to me!
a year latter,they sent me an invitation to the centurion card, i accept and then closed the platinum account; they deleted the guarantee right after I closed the Platinum, now my centurion card is unsecured!
Maybe they used this, in order to create history with Amex and then, if you do not fail to pay them, they delete the guarantee!