I did choose to use my own email when contacting their support once, but they never asked for my account info. They are a paid service so it isn't like they really need to know if you are a customer or not (like companies that may offer free accounts without support)

Plus, I could have used a burner, other measures, etc. You can and probably should pay privately and generate a new account number every renewal anyway. You could also do this before contacting support and then go back to your actual account number if they did need one for some reason.

There are too many options for Mullvad that I don't know of for any other provider. They allow you to be as secure and private as you care to be, or as convenient if that is your choice.