$f\sim g$ does not imply $f'\sim g'$! L'hopital's rule only works in one direction:
$$\log x \sim \log \left((5+\sin x)x\right) \quad\text{but}\quad\frac1{x}\nsim\frac{((5+\sin x)x)'}{(5+\sin x)x}$$
or if you want,
$$\log\log x \sim \log \log \left((5+\sin x)x\right) \quad\text{but}\quad\frac1{x\log x}\nsim\frac{((5+\sin x)x)'}{(5+\sin x)x \cdot \log\left((5+\sin x)x\right)}$$
(The factor $5+\sin x$ is there just to make the second quotient misbehave.)
The point is that we don't know (a priori) that
$$\frac{\pi(x)}{x/\log x}$$
has a limit for $x\to\infty$.
What l'Hopital does tell us, is that if the limit of $(\pi(x)\log x)/x$ exists, then it is $1$.
I believe Chebyshev's original proof (and any subsequent ones) of this fact also goes along these lines, via a Mertens-type estimate for $\sum_{p\leq x}1/p\sim\int_1^x\pi(t)/t^2$.