If the plane was on descent, at 6000 feet it would have been traveling at 450 km/hr which is only 125 metres/sec and contrary to popular belief, at that speed is is no problem to see objects and focus on them if they are spotted well out in front or to the side.
The pilot saw something - but what it was is the issue.
I believe it's highly unlikely that it was a drone up at 6000 feet.
Pilots read these stories all the time and many have no idea of the performance limitations of recreational drones.
But having the idea that the air is full of rogue drones helps their mind to put a label on indistinct things they might see.
In the 1970s and 80s, such things were labeled as UFOs and pilots frequently reported them.
(Almost) no-one believes there are flying saucers up there these days, so they get called drones now.
The US FAA did collect such incidents and made them public for two years.
Journalists had a great time making headlines about the dramatic increase in drone/airplane interactions and how it would lead to disaster.
Here are just a few questionable records from the FAA database which has several dubious records every week.
How many do you believe?