Yes, I have seen these claims. In fact it came up not too long ago though I don't recall if it was on this forum or another one. But I found some research that showed it did contain vitamin C. The amount is what is more in question, but what they need to keep in mind is that the vitamin C in amla was found to be 12 times stronger than the unstable synthetic ascorbic acid commonly sold in stores. So large amounts are not needed as with ascorbic acid to get the benefits from the vitamin C.
By the way, it is those antioxidant tannins that make the vitamin C in amla so stable. Basically there is an antioxidant protecting the antioxidant.
And yes, these tannins do have other beneficial properties, such as antiviral, antibacterial, and antifungal. On the other hand I don't recommend combining the standardized extract with other herbs as tannins can lock up the active constituents of other herbs rendering them useless to the body.