GIMP still lacks non-destructive editing, does it not? That alone is a huge deficiency in comparison to Photoshop.
I think the issues are getting confused. Yes, Lightroom saves a separate file from your photos of the changes so in that sense it's non destructive, the original files are not changed.
Photoshop is non-destructive in that you can load an image into a layer and apply tons of layers above that layer each of which edits the layers below it in some way. The pixels in the image layer are never changed so it's also "non-destructive". But saving the file does overwrite the old photoshop file with your changes, unlike lightroom.