According to this article, 11.6% of 959 healthy volunteers in a random sample were positive on their antibody test. so 12% of italians had had Covid in September? And antibody levels decline after a while, so probably even more.

Sorry, but this would be pretty insane. Most likely explanation is probably false positives or scientific fraud or a combination. Antibody tests don't really prove much anyway since they are not directly identifying the virus. A full sequencing of a virus would be much better. The RNA would also give us information about the evolution of the virus.