How does one even accidentally steal a texture someone else made?

One could easily apply Hanlon's Razor to this. For example:

• Bob is a game artist. Bob has a folder on his desktop called "Inspirations", where he saves art pieces he finds online that he likes, and a folder called "Assets" where he saves things he's created for the game. Bob transfers to a new department in the studio, or quits, or is fired; either way, he returns his equipment to the IT office.
• Dave is an IT guy at the studio. Dave takes Bob's computer after he leaves the job, and transfers all of Bob's files to the studio's shared drive. Dave isn't an art guy, and doesn't know the difference between "Inspirations" and "Assets", and dumps them all into the shared drive in a folder called "Bob's Things".
• John is the studio's new artist, replacing Bob. John syncs "Bob's Things" to his computer. John assumes everything in this folder has already been cleared for use by Legal. John starts implementing the art into the game.

I used to be a pretty hardcore Destiny 2 player for several years. In that time, I've seen Bungie fuck up a lot of things. But those fuck-ups were almost entirely caused by somebody in the studio not playing close-enough attention to something, and details getting mixed up in the pipeline. I don't think anybody at Bungie knowingly put Antireal's art into the game. I think the more likely explanation is that there was a lack of oversight, and files that shouldn't have been mixed together, got mixed together.

It wouldn't even be the first time Bungie had something like this happen; there was an instance where a third-party studio that Bungie contracted to build a Destiny 2 cut-scene accidentally used artwork that was not intended to be in the actual cut-scene.

Not to suggest that any of this excuses Bungie for multiple cases of plagiarism. Obviously, they need to have stricter standards in place when transferring files between parties. It's a colossal fuck-up, but I don't think that it was a fuck-up anybody set out to commit.