Thank you to the contributors here - this has been a very helpful discussion, directly relating to the same experience with the same direction by SOCAL for our aircraft heading west.
Where the confusion lies (for international operators like us) is that there are MANY non-US FIRs around the world where there is NO speed restriction for IFR aircraft operating in Class C airspace below 10,000'.
So the ATC instruction, "cancel speed restrictions" is common and permits speeds greater than 250 KIAS below 10,000'. In Australia, we are routinely permitted (with ATC explicit instruction) to exceed 250 KIAS below 10,000'.
The reason ATC still has to "cancel the speed restrictions below 10K" is that the SID/Departure or STAR/Arrival Chart will specify "maximum speed below 10,000' is 250 KIAS".
On the most recent departure from LAX heading west, we were given the same instruction to "
"Maintain 280 (knots) or better, climb and maintain XX,000"
We, evidently incorrectly, immediately accelerated to 280 KIAS - instead of waiting to reach 10,000'.
This flagged on our FOQA and a robust discussion was had.
Anyway, we had already determined that the best course (in US airspace) was to delay the acceleration until 10,000' - which we'll do next time. 