On that really bored, the opto couplings are set up for 12V, when they are pulled low. which means that the current is limited for 12V, not 3.3V.
The 16 channel board has the same limiting resistors for the 12V and 5V relay versions. The resistors are connected to the on-board 5V regulator. I have not seen any reports of the 16, 8 or 4 channel relay boards not working when the opto is controlled with 3.3V source voltage. The opto current at 3.3V would be about 55% of what it is at 5V (allowing for the 1.2V IRLED drop).