For what it's worth... you just taught me something. I didn't know I could even get 'Seller Hub' on my phone. But it appears just as you explained... through Help.
With that being said, I don't know how else you'd get to it. 😐
The app has its own selling interface which is distinct from Seller Hub.
If you want to use Seller Hub you can use a browser in desktop mode, or, as you have found, you can open a browser window inside the app by going through the Help page.
