Why do you have a separate checkbox for "I'm a freelancer/gig-worker", and a separate checkbox for "self employed" if being a 'gig-worker' is considered self-employed?

Most likely there is a separate check-box for 'freelancer/gig-worker' and 'self-employed' because not everyone who is a freelancer or gig-worker is aware that they are self-employed.  

If this is their first year doing that type of work they could recognize that they are a gig-worker or freelancer, but when another company is paying them to do that work, not everyone realizes that if the tax document they receive is not a W-2, then they are not an employee of that company.  

Doing taxes and knowing all the terminology associated with it is a learning process and TurboTax tries to help users learn along the way.