That doesn't necessarily mean they have your weight. A sensor that triggers at a fixed weight is gonna be waaaay cheaper than an actual scale that can maintain any degree of calibration over a decade as people plop into their seats.
Discussion
The article I linked links to a NY Times article on this. But its behind a paywall. This, I believe, is the same article. https://www.nytimes.com/2019/05/20/opinion/car-repair-data-privacy.html
It says "Cars not only know how much we weigh but also track how much weight we gain. They know how fast we drive, where we live, how many children we have — even financial information. Connect a phone to a car, and it knows who we call and who we text."
I love/hate how informative Naomi Brockwell is. Here is more information on how nefarious it really is:
Why do they need to know our sexual activity?