I have heard the debate myself, but I seriously doubt that they would have mistaken a vitamin C for a tannin. But I looked up some studies and it still appears that most of amla's activity is from vitamin C.
Vitamin C content and antioxidant activity of the fruit and of the Ayurvedic preparation of Emblica officinalis Gaertn.
Emblica officinalis Gaertn. is one of the most important plants of Ayurved, the traditional Indian medicine. In this ancient medicine, the fruit of Emblica officinalis is processed according to a method named "Svaras Bhavana", whereby the therapeutic potential of the plant is enhanced by treating the main herb with its own juice. For many years, the activity of the fruits was attributed to the high content of ascorbic acid; however, this has recently been questioned. The aim of the paper is to clarify this matter. A reliable and feasible HPLC method with diode array detection has been developed for the determination of ascorbic acid in Emblica fruit and particularly in Emblica fruit processed according to the Ayurvedic method. The antioxidant effects have also been evaluated in comparison to the real levels of Vitamin C by different antioxidant tests. The data obtained show that the Emblica fruit contains ascorbic acid (0.4%, w/w), and that the Ayurvedic method of processing increases the healthy characteristics of the fruit thanks to a higher antioxidant activity and a higher content of ascorbic acid (1.28%, w/w). It has also been found that Vitamin C accounts for approximately 45-70% of the antioxidant activity.