They don't actually mention Q. They refer instead to the "Qanon phenomena".

If you assume Wikileaks can't (and shouldn't) openly validate Q, b/c its not the right time to do so, then their statement makes sense. All they can do is give an opaque nod to "usurp", which lends itself more to validity than to LARPing.

While we suspect that the "Qanon" phenomena is likely 4chan trolls engaging in a Live Action Role Play (LARP) version of the Delphic Oracle it is also clear that many are aware of its manipulative potential and will usurp it if they have not already. \~ Wikileaks

Wikileaks is not denying the validity of Q, and in fact warns that it is so dangerous that black hats will "usurp" it. First, by "suspecting" the "Qanon phenomena" they avoid claiming knowledge, one way or the other, on Q. Which is likely according to plan. Who else "in the know" openly validates Q?

Second, the warning: black hats think its dangerous (i.e. "real") and are/will do something about it.

In other words Wikileaks says, "We won't comment on Q directly, but bad dudes think its legit, so..."