I was confused about this too but I figured it out by sifting through hints in turbotax help. It's not obvious.
The savings bond option only shows up in that list if you choose to pay turbotax fees with a credit card, instead of deducting the fees from your federal refund. (If you've already chosen to deduct, you can go back to step 1 in File, Revisit it, and change to credit card).
If you deduct from your refund, only the direct deposit options and the check option are shown in the list.
If you pay with a credit card, a new option shows up: split between multiple accounts or buy savings bonds.
