The formula for speed is Speed = Distance / Time
If the distance is in question, then the speed is also in question.
The situation is fluids traveling within fluids. The winds and water are in motion. On board airplane airspeed indicators are similarly unreliable, and are not used for navigation. The local area of the airplane might be in motion faster than a larger area around that airplane, which might itself be in motion.
This is a joke, right? Apart from their vital use at the lower end of the range, around stalling speed, this is the main reason for having an airspeed indicator. I don't know when or where you did your own pilot training, but when I was taking lessons (in 1973) the first piece of kit I had buy was something called a "computer", which was a type of circular sliderule, where you would factor in your airspeed, intended course, and wind velocity and it worked out the heading you needed to steer. En route, similar inputs would tell you how far you had travelled. This is why commercial aircraft and large military aircraft carried navigators. The windspeeds are forecast by meteorological agencies, and verified by shore stations and weather balloons.
The advent of more advanced terrestrial navaids in the 50s, and GPS in the 90s has reduced dependence on dead reckoning, and consequently reduced crewing requirements, but its still a vital part of crew training. Its also how Amy Johnson, Jim Molinson, Alan Cobham and thousands of other civilian pioneers and military pilots managed to find their way around the South Pacific in the 30s and 40s.
And I'd be very surprised if the Vendee sailors aren't comparing at least 3 different forms of navigation.
(Incidentally, leading boat currently has a windspeed of 23 kts, to the rear of the peloton its around 8 kts. Scary stuff).