You are absolutely free to use Osso's source to onboard and authenticate your own customers without paying us! The goal of the license is to prevent someone from creating their own SAAS offering that competes with ours. Using Osso for your own app is within rights granted by the license.
But to your point, we also offer a demo instance that you can use in order to build out your integration, and then when a customer comes along, start paying us and swap out some ENV variables and you're ready to go!
In the mean time the free version lets me learn and experience the full product.