> Photoshop's content-aware fill first appeared as a third-party open-source plugin for the GIMP.

Indeed it did, and that plugin was hard to configure and hard to use. I remember installing GIMP just to try that plugin out, and it was not fun, it involved a lot of manual steps to get it working, and I never quite got the hang of it.

In contrast, when I got my hands on a version of PS that supported content aware fill, (which admittedly ships in the box, skipping the installation step) I was able to figure it out in a few minutes and start getting good results in half an hour. I am pretty sure I spent more than half an hour trying to get the GIMP plugin installed.