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/
Pretty sure that is entirely an unfair comparison. You don't need a connection per thread either. It is common to have web servers handles thousands of requests per second with a connection pool size of 20. As long as the handler threads borrow a connection for only a little time and block waiting for a connection while other threads are doing their thing, it works out fine.
There is absolutely no logical reason why database pool sizing could be a reason for preferring fibers over threads.