Contamination wouldn't occur due to the spacecraft, but due to handling in the lab. While I'm not an expert on organic chemistry, I think it's pretty unlikely that organic contamination would ONLY include uracil. Scientists spend a lot of time making sure their data is significant. Additionally, this isn't the only research group with Ryugu samples that has found complex organics, though I think these might be the most complex found so far.

Source: I work with Ryugu samples. Take my opinion with a grain of salt as I'm still in grad school.