If you read the information in that link, there were crafts in our atmosphere beginning in 1947, which had capabilities that outperform our current (publicly disclosed) most advanced aircraft. The performance capabilities included speeds of thousands of miles per hour, 90-degree turns, instantaneous acceleration, and silent operation.

Many of these sightings around the world were accompanied by radar which confirmed the visual observations. Some have left detectable levels of radiation. All of this is cited in that link. Also in that link is the Nimitz Event, in which a UAP outperformed F/A-18F Superhornets and left the Top Gun pilots shaken.

I'm not stating this is technology from outside our planet. I'm not saying this can't be human technology. Look at Project 1794 in that link. That was the United States Airforce's secret contract to Avro Aircraft to build a literal 'flying saucer' powered by 6 Siddeley Viper turbo-jet engines.

It's important to note, however, that the aircraft from Project 1794 was not capable of the performance capabilities of the UAP/UFO that began being mass sighted around the world in 1947.

So either we have kept aircraft technology so far advanced that it outperforms modern declassified tech secret since 1947, or something else might be responsible.