If the startup service is using the internet service, and the Internet service is prioritized under the telephone service, indeed it will potentially be slower, but not because the startup is being discriminated against. It would be slower because ALL Internet traffic over that link is discriminated against.
The most likely reality is that any additional services will have a minimum QoS set that is required for a reasonable experience. This would only be hit if the customer premise is utilizing the entirety of the physical connection...essentially they're providing guarantees that TV/Phone/Internet/etc. would work at at least a certain level of quality, which would be at the expense of other services based on what those minimum level of qualities were defined as.
And while today this may sound weird (we are used to have phone connections via our ISP) what happens if tomorrow's technology (say an interactive film) ends up in the same situation? A startup creates a field, just to get squashed by the ISP who picks up the idea (that clearly works) but gets extra priority because it is their own service.