If they are doing that, it is the way they have chosen to interpret the guidance, it is not what the actual guidance says. As this article states, whether or not you can use your car in order to exercise is not mentioned.
https://www.bbc.co.uk/news/uk-52062209
The intention of the new rules is to minimise human interaction so why would you prohibit something, which is totally consistent with that aim? The rules should be a means to an end, not an end in themselves.