The open source version is fully functional and can definitely scale beyond toy projects. You don't get high availability, multi-instance clustering, auto-scaling, multi-region failover etc. built in, but if you put it on a beefy host it can easily handle a large number of users and a very high request rate.
The way I think of it is we give you the fully functional server, but charge for advanced infrastructure and a few advanced features (SSO/Teams).
It's comparable to the open source version of a tool like Vault where you get the server, but need to implement advanced stuff like HA, auto-scaling, networking, etc. yourself, or else use a paid version.