I did not know that cleanrooms have classes. Apparently they used a 10000 class cleanroom which is one of the "dirtiest" grade.

Surely an astroid sample return mission is scientifically one of the most important and difficult things to accomplish, and I wonder why they did not use a higher class cleanroom for this even though they mentioned "Researchers recommend enhanced contamination control procedures for future sample-return missions to prevent microbial colonization and ensure the integrity of extraterrestrial samples."

I bet the hardest part is flying a higher class clean room equivalent to an asteroid, so it lands immaculate on Earth. Otherwise it doesn't make much sense to upgrade the clean room when the sample has been exposed already.