I'm referring to the thing the attacker computes right before that sentence. The attacker has computed:
C_1 ^ C_2 = P_1 ^ P_2
The right hand side is known; it's the two ciphertexts. The right hand side is hence also known, but neither P1 nor P2 is known. Except, of course, if you have a plaintext that has a lot of Z's in it, because then I know one of P1/P2, and I can cancel out the Z's :) Is that what you mean by "indirectly"?
Wouldn't it reveal the stream of bytes the plaintext was XORed with, not the plaintext? Unless the article means indirectly, which would be strange writing.