It can if you control the training data, or if the data is public domain.

But I get your point. You can say all the Diamonds you use are conflict free, but out of the thousands you have, how do you know some have not slipped in.

At what point do you say you did a good enough job, and at what point is it too contaminated?

Like many things, it difficult to draw a line, so its up to communities to set a reasonable standard.