Yes, that's right, all GMs must sign up to the OOOL to join the league.
If you want to run the league the same way as the standard leagues do, it makes no sense to make it an OOOL league
The OOOL offers additional features (and the plan is to add a lot more additional features) that require the connection to an online database. That's why all GMs have to sign up.
If you want to see your league in a leagues directory, I suggest to submit it to the IOLD, which is the greatest and most complete leagues list AFAIK.