One reason I think it could be MPH despite that is because some of the other data seems like it's been processed so that it doesn't need to be transformed any further on the client side before using it in the UI, and the UI displays the speed in MPH.
If I were still on the flight, I could just compare the numbers in these payloads to the MPH number in the UI and confirm.
Based on the lat/long of your destination and the coordinates of the plane I believe the distance and speed actually are in miles and mph: https://news.ycombinator.com/item?id=37694487
Well, most airlines. I think both China and Russia already switched to SI units (so km/h), and supposedly ICAO recommends using km/h but there is exception for using knots and there is also no end date to stop using knots, so everyone just continues to use knots.
It's not that they 'already switched', but rather that early Russian aircraft had used the metric system for instruments and China acquired much of their early aircraft from the USSR.
In the West, it was well into the 50s before knots became conventional. Many (but not all) British and American aircraft used miles per hour, and most of non-communist mainland Europe used the metric system. I am not aware of whether there was some agreement to choose knots, but by the 60s almost all western aircraft had instruments in knots and nautical miles.
