Supercar car makers each have a different view of what constitutes being a 'production' car. I know what the FIA requires and I doubt any supercar manufacturer ever has three cars being assembled, at the same time, to a single specification.
I have not read of any independence in the assessment of speed, although anyone can hire or buy a Racelogic V-Box from the maker.
Some reports do talk of an 'average' speed, which can only have been achieved by data manipulation within the V-box. Tyre circumference will differ as the speeds change so the distance would have to be taken from two 'instantaneous' GPS measurements of the vehicle position.
If the speeds were 'peak speeds' then these supercar factories are writing their own 'rules' for what amounts to a multi millionaire wall p*****g contest. An interesting exercise but not record breaking as the rest of us understand it to be.