They already had the replacement assets created upon release, but forgot to swap them out and it was missed in QA. That’s why, pretty much as soon as they discovered it, they got it all replaced just 5 days after release.

It’s pretty common for games to mistakenly release with minor assets still as placeholders instead of the official version. Who’s really going to look at the texture of a rock and be like “Wait just a minute! That’s not the official rock texture!”