The law hasn't changed in the last 10 years to my knowledge.
However, according to the maps I have (I can't immediately find an online equivalent) the road up towards the Wong Shek pier is not designated as country park. The road is restricted access, but it is not actually in the park. To the east of the road you have Sai Kung East Country Park, and to the west you have Sai Kung West Country Park.
Hence, when walking a large dog along the road it would need to be on a leash since it is not in a Country Park. However, as soon as you take any of the trails off to the side you are entering one of the Parks and the dog can be let off the leash. I would expect there to be signs on each trail that crosses the road indication that you must leash your dog as you approach the road (they should, therefore, be facing primarily away from the road).
The law as I quoted it above is clear. Inside a country park dogs may run off the leash. This is further stated explicitly here on the AFCD's page regarding the Exemption Examination which your large dog can take to be allowed to be off the leash in other public places as well.