That's for inter-island. They definitely do advance seat assignments for the west coast to Hawaii flights. If Hawaiian Airlines is telling you that none are available, that means the flight is pretty full so you need to manage this carefully.

If after calling HA a few times prior to your departure you still don't have assigned seats, I strongly suggest checking in online at as close to the 24 hour mark as you can. If you still can't get seats at that point then be at the airport no later than 2 hours in advance, as the later you arrive, the worse your seats will be (as in all middle seats, in different areas of the plane).

If you have assigned seats before arriving at the airport (either advance or via online check-in), arriving 90 minutes prior to departure should be fine. It can take a bit of time to get your bags dropped off and get through security, but at terminal 2 at LAX 90 minutes will be plenty of time.