Two data points using the CS Platinum and Vanilla Platinum:

CS Platinum:
1/10 - Bought Delta gift card on delta.com for $50
1/13 - Purchased flight for $267.80 using the gift card and charging the rest on the CS Platinum
1/17 - Meter on Amex account page says "Congratulations! you've received $200 of your Airline Fee credit benefit this year!"
1/18 - Credit for $200 posts on Amex account
1/18 - Cancelled flight in exchange for $267.80 eCredit

Vanilla Platinum:
1/10 - Bought Delta gift card on delta.com for $50
1/13 - Purchased flight for $251.80 using the gift card and charging the rest on the CS Platinum
1/17 - Meter on Amex account page says "Congratulations! you've received $200 of your Airline Fee credit benefit this year!"
1/18 - Credit for $200 posts on Amex account
1/18 - Cancelled flight in exchange for $251.80 eCredit

Thanks as always!

P.S.: Friendly reminder to keep this usage of a credit on the DL/a need-to-know basis. The other day, someone made a post on the r/amex subreddit blabbing how they received the credit with this method, prompting several people in the comments to ask about how they can use it also, along with lots of angry comments telling the OP not to broadcast it publicly. If anyone on social media asks about how to use the credit, simply refer them to this website or tell them that "Google is your friend." Posts like the one the other day increase the likelihood of endangering "the method."