the devs thought it was ok to put it into their game

That's the point. They didn't thought it was OK and didn't.

They could have just used stock textures as placeholders like developers have been doing for decades.

That is exactly what they did, any texture left in the first version of the game was a mistake that was promptly fixed as soon as they noticed it. We have the advantage of judging four years later with new info something they did back then and have since corrected. Ethical considerations must include intent and context, and here there was definitely no intent to harm.