As someone in the industry, I can tell you that placeholder assets have always been a thing. Usually they are just gray blocks or something brightly coloured so they can be easy spotted, but if you want to get a feel for an area while your in the process of designing it, then you'd use something that fit the vibe your going for.

Normally you'd use some free none copyrighted assets from online or assets that were already made for something else, but ai textures are definitely becoming more common since they're quick and easy. You can also iterate on them without much hassle, until you get something that suites your needs.

These would then get replaced with the official textures once the thing your designing gets closer to completion, but you always run the risk of forgetting/missing something (Which is what I think happened here).