I don't think it will affect much even if they are found to be guilty. It is a known fact that BitMex occasionally trades against the users, and still, they have a huge volume of trades. Even this news didn't affect the trade volume much.
Those accusations are huge and if true it could be enough for the authorities not only to go against the people at the top but against the whole business as well, as it is obvious that such thing must be illegal as an exchange should be impartial and just provide a platform in which traders can trade against each other, if the exchange takes action on their own platform they have an unfair advantage over the rest of the traders since they have information they do not and if they use that privileged information then they could be accused of a crime.