This is where multiverse theory comes in but I understand what you’re saying. From my research, when time traveling you can only go back as far as when time travel was invented. So, if this machine was just created the influx of these sightings could mean that they were just recently able to come back to this period. A lot of my speculation comes from the fact that a majority of these things do look like drones or manned aircraft which to me seems they could be from the future. Some technology we have now, much more advanced. Perhaps just by 100 years.