It probably is comets, but why the comets are in the position they are in is not well described.
Science is done by people, and it is often very challenging or impossible to rigorously quantify exactly how unlikely something is. This is especially true in observational science. Furthermore, in communicating results to the public it isn't necessarily helpful to speak with quantitative rigor because the audience doesn't care. If someone says this has a 1 in a 10000 chance of being due to aliens, the headlines will scream "Scientists say alien life could be found!"
Science is done by people, and it is often very challenging or impossible to rigorously quantify exactly how unlikely something is. This is especially true in observational science. Furthermore, in communicating results to the public it isn't necessarily helpful to speak with quantitative rigor because the audience doesn't care. If someone says this has a 1 in a 10000 chance of being due to aliens, the headlines will scream "Scientists say alien life could be found!"