Actually after digging through the massive pile of posts in the Amex forum about the $200 airline credit, it appears that Amex would NOT apply the credit to the buy-mqm promo. The issue is, DL codes the transaction the same as buying miles, which Amex says is not eligible for the credit.

Buying skyclub membership is supposed to get the credit, since it comes across as a membership fee and not miles. Current promo for that is buy a year and get 1500mqm bonus.