With the DL Amex, the benefits accrue to the traveler's SkyMiles account while OP holds the card. In OP's example, OP won't have a DL Amex on the date of travel, so benefits would not be available.

Conversely, if OP were to acquire a DL Amex in Jan 2018 for example, DL Amex benefits would apply to a Feb 2018 trip - even if the trip was purchased prior to holding any DL Amex.