I was thinking the same as parent while reading this. Mentally this activates the same thinking as on those medical tests with a high false positive rate and low incidence, so that most positives are false. I'd like to see in the article how they rule this out. Ideally I'd like to hear that they have measures in place that would allow accidental lapses in isolation to fail and they'd still be able to tell that it was Earth contamination. It's a reasonable concern and having it addressed (with something more satisfying than "they're experts, duh!") makes this kind of finding all the more interesting.

I mean, it's a concern, but there are numerous other odd things in the findings that would not be caused by ground contamination such as the amount of stardust contained in these samples versus other asteroid samples, or the huge amounts of clay/water created minerals found so far.

There are plenty of other articles on the isolation procedures they've taken so far to this point including putting off opening the container for months because of a stripped screw.