McLaren Senna (P15) lap time at Silverstone GP (post 2011)

If you infer, you will note that the Senna beats a Ducati GP14 by 10sec on Silverstone. The GP14 achieves a 2.04 on COTA, 13sec faster than a McLaren P1. Looking at the layout of COTA (a lot more corners and heavy breaking), I would expect the margin to the GP14 to be wider... meaning that the gap to the P1 is even more vast. And yet Chris Harris makes it seem like the McLaren Senna serves no purpose whatsover. For a car to blitz the holy trinity (similar priced cars) by such a margin is an exceptional feat. And this is a car that you drive to the track, does not need an instructor nor track-prepping.

There is clearly some mishap with MotoGP times as there can't be 18 second difference between 2016 and 2017 unless the 2016 qualifying session was in pouring rain.

Whilst 2:09 is certainly a decent drive it is by no means the limit of performance of the car.
The purpose of this test was to show how much faster a Gt3 car is vis a vis the senna. Chris harris doesnt take into account the huge amount of seat time he has in the GT3 car when he has relatively no laptime in a yet to be broken in fully senna.

The fastest MotoGP lap at Silverstone is 1’59.941” set by Marquez in 2017 during qualifying. The 2018 race was cancelled due to persistent rain.

Yes you're right it was a rain affected race hence the 18 second lap time difference.