Knowledge of discrete log is needed in the proof of Cramer-Shoup public key scheme?
Of course, it doesn't mean that adversary knows how to solve discrete log. 
We just wanted to say that adversary only knows that the point $P$ lies 
somewhere on a plain $\cal{P}$ of such form.
We don't suppose that adversary knows actual parameters of the plane.

But it's enough for us to deduce that $\cal{P}$ intersects with another plane $\cal{H}$ (which also has a specific form, though we don't know exact parameters) only by a line, so this intersection is negligible (actually, 
a probability for $P$ to lie exactly on the line is negligible).