Northbeach - if you don't get an answer from anyone from SoCal ATC here then give them a call and find out, best to be knowledgeable than guessing. Otherwise, next time you get a similar clearance then ask whether that is approval to accelerate at your current level or wait to 10,000', you certainly can't be wrong for ensuring you are doing correct in such a safety conscious environment and may highlight to the controller concerned their need for more preciseness in future.
Here in the UK we use the addition of "unless authorised by ATC" to that speed limitation, which I think is far more clearer. I would personally expect you to accelerate to 280kts at 6000' as long as your airframe permits this at that level.