When you request a taxi you get a private hire email... I assume that element is fully legal. They also seem to process the payments in HK so you don't get the 1% surcharge when you use a taxi.
I believe the other private car hire is only strictly legal for a small quota of vehicles (i presume private limo and airport/hzmb transport type companies?) and in reality none of the ubers are part of that quota.
Lai King, West NT to Hung Hum, East Kowloon? No. Frickin'. Way.