I can almost guarantee you that what happened is that Antireal’s art was saved to one of the artists’ “inspiration” folders, and somewhere between creating an inspo gallery and that artist quitting/getting laid off, the files got merged into a “concept/assets” folder. Likely when going through the laid off artist’s hard drives, they found images that they probably assumed were all originals.

I've worked in games for 18 years, and outright theft of assets happens constantly. I worked on a game where an artist took an asset (from a rather well known game) and outright used it, no changes. Another place I worked an artist took an (extremely unique) weapon from an IP and just copied it over.

If you look at the Bungie art it couldn't possibly be an inspo gallery because they are just straight up used with (some) changes made to them. This was absolutely known to have occurred, and wasn't 'an' artist. You can see why they do this because even in the event they're caught, they can blame a former employee, and pay some nominal fee for the actual artists work.