No, this restriction has always been there but the credit worked for gift cards anyway and wasn't clawed back. Until now, when someone reported that their credit was reversed due to having purchased a gift card.

If Saks reports third level data then Amex IT should be able to block transactions for gift cards from receiving the credit. Apparently that data isn't being reported to Amex because gcs have easily gotten the credit ever since the introduction of this credit.

With the airline fee credit for most airlines Amex is able to differentiate when a ticket is purchased outright and it usually won't trigger the credit (I know that for JB(B6) and SW it does for low priced tickets, but overall they are able to exclude ticket purchases from getting that credit).