Because unless it is a forgery, it is signed by a WADA official.
Now it could be photoshopped, anything in this world is possible. But my Spousal Unit wrote a book on Photoshop, and I asked him to look at it, and he saw no evidence. ETA: I'm not saying it's a forensic investigation, but I tried with what I had
So far, it is the most trustworthy source. Certainly more trustworthy, as facts, than Kamila's lawyer. (I don't blame KV's lawyer, that's what lawyers do. She's doing it. Good for her. But it's not the best or last word on anything.

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