How are there '16 plate Land Rover Defenders?

The plate is based on the date of first registration of the vehicle.

Point to note, before a vehicle is registered it is classed as goods and moving brand new un-registered cars on a truck with trade plates is not allowed - this was first prosecuted in Canterbury iirc...

As Solar Mike said, it was about when they were registered as "on the road" with the DVLA. You would be given an age-related plate if the vehicle was imported or released from the military. It would be registered, say 2020, but would get the plate from when it was first used (first registered in the original country or entered service with the military)

There are a few dozen Defenders on 66 plates, with many registered on 28th Feb 2017, which was the last date of the 66 plate.

There's even a Defender on a 17 plate, but that was built in December 2013 and was stored unused before being converted into a V8 70th Works model and registered in 2017.