Upvoted for precise thinking.

I agree that part of the problem with "if it can be known" is that "if it be known" is strictly superior.

On the other hand, an important fact in the critic's argument is that the consequentialist agent knows that strategy Y leads to horrible consequence Z. If he is simply unaware of this fact (but "it be known" by other people), then we would be entirely unsurprised to see him choose strategy Y - he is still a consequentialist, just a misinformed one - so no argument materialises.

Therefore I think that the use of active voice is clearer in this instance, because passive voice obscures the distinction between our knowing that strategy Y leads to horrible consequence Z, and the hypothetical consequentialist in question knowing this. Admittedly this distinction is not difficult to infer in the original piece, but numerous small inclarities can add up to make tedious prose.

On reflection, what you have said about the passive voice in general is true. Misuse of the passive voice to obscure agency when agency is politically or socially important is Orwellian; passive voice in general does not necessarily obscure anything.