The reason you need the Device Setup SDK for Wi-Fi devices is to set the Wi-Fi credentials on the device, which is often easiest with a mobile app. (Using a browser based setup on the Photon and P1 with SoftAP, and a hardware solution like a display and keypad, are the other options.)
In order to communicate with the device you set up, you can use the customer account feature, either using simple auth or two-legged auth customer accounts.
For cellular devices, there are no on-device credentials to set. Cellular product creators either build the monthly fee into the cost of the product, or have their own recurring billing system. Therefore, creating a parallel system also using the Particle authentication system rarely makes sense.
For cellular devices that the user interacts with, most product creators have their own system, either web apps from a browser, or their own front/back end system, possibly using a mobile framework (React, Ionic, Flutter, etc.). It doesn’t make sense to also add Particle authentication on top of this, when it all can be handled by your back-end server more easily.
The Particle app for Android actually includes the Device Setup SDK for setting up Gen 2 devices. It also has the system for setting up Gen 3 non-product devices. You could merge the two parts together and make a custom app to create Gen 3 product devices, but this isn’t supported or documented.