Aren’t Private Companies Like Ride-Sourcing Entities Exempt From U.S. Department of Transportation (DOT) ADA Requirements?

No. The DOT ADA regulations cover transportation provided by both public and private entities, whether or not they are primarily engaged in the provision of transportation service.
For example, if a hotel wants to provide shuttle service to its guests along a fixed route serving local attractions, because hotels are not primarily engaged in transportation, the vehicles used may not need to be accessible as long as equivalent service is provided for persons with disabilities, including wheelchair users.