You CAN use a single database instance or file for everything, you can also use multiples to scale without falling strictly into a heavy vertical or horizontally scaleable database system. Especially if those db instances are distributed over a network channel to different physical servers on the back-end (as is with Turso).
I've worked on a lot of systems where I had advocated the use of separate DBs either on a oer-client or per-project basis... Even from a single management server, you can do a lot... from a small cluster operating against SQLite databases you can do more. With Turso's efforts to improve concurrency, it can go further still.
I'm not an employee, related to, or even an active customer right now... but I do understand the model and how it can work in a lot of use cases.
Sounds like these are different ways of manually sharding the database?
And yeah, it's effectively sharing data... My point is there are lots of use cases that can fit in this type of database usage.