So they found non-existing references in 0.1% of 2.5 million papers examined, and only 0.01% contained more than 2 fake references. Their method also appears to have had a false positive rate for fake references of 7/10.
While the problem is concerning, and growing (fuck AI), 99.9% of papers are ok. We have always known we have to read every paper critically, now there is just an extra thing to look critically at.
In my experience, a way bigger issue is references that don't actually contain or proof the information for which they are referenced.