I think you misunderstood that sentence. Formbricks is Open Source under the AGPLv3 license and always will. It's not "Source available, it's open source.
And to be able to maintain the open-source codebase in the long run and finance the work we and other team members & core contributors put into the product, we are developing features beyond the Open Source version that we charge larger businesses for. But this doesn't affect the open source version under AGPLv3, just makes it better because we can pay a team to maintain this.
From the second line of text describing their open source, self hosted version:
"(In the future we may develop additional features that aren't in the free Open-Source version)."
Not exactly a statement that gives great confidence in their commitment to Open Source.
It would be nice if these types of companies marketed themselves as "Source Available, cheaper than X" rather than 'open source'.
Open source feels misleading if you intend to fork it from the beginning.