Thanks for the explanation, MSPEconomist. It makes mores sense when considering that pre purchase you are actually paying the difference in fare with miles (in this case L/L to I/D) for $194 or 19,400 miles each way (caught me rounding in the OP).
It seems the restriction still holds - can't pay a portion with miles and a portion with gift cards or ecerts. Is that an IT quirk or an intentional restriction on Delta's part?