Carmine (which coded this Fibers update) wrote about this in his blog. See the section 'The database connection math'. And no, you won't have one connection per fiber.

The difference is staggering when you compare to threaded mode: it requires 1,320 database connections to run the same benchmark that the fiber mode runs with 60.

https://paolino.me/solid-queue-doesnt-need-a-thread-per-job/