While SBD are worth more than STEEM, you can enroll with SBD (and we will treat the surplus as a donation). We wrote it that way to make it clear that if you send SBD instead of STEEM, we won't refund the difference.

When SBD drops below STEEM parity (more than 1 SBD to buy 1 STEEM) then we will start returning SBD transactions unless they are specifically marked as donations.