How does one […]

This is the key element. I don’t think this is a case where a team collectively chose to steal someone’s art.

If the theft was deliberate, it was probably an individual, with how big these projects are it’s not hard to consider how that may have flown under the radar.

I could also see one team member collecting assets to serve as inspiration and another implementing them without realizing they weren’t created in house.

With how exhausting the current state of the world is, I could even see a burnt out employee tossing something together without remembering where the asset came from.

Not trying to excuse what happened, the original artist is definitely owed for this, but there are other potential explanations for this beyond intentional malice.