de Jong's conjecture on isocrystals over simply connected varieties

Let $X$ be a smooth projective variety over an algebraically closed field $k$ of characteristic $p>0$, and let $\pi_1(X)$ denote its étale fundamental group. An isocrystal on $X$ is a $p$-adic coefficient object in the crystalline framework. de Jong's conjecture. If $\pi_1(X)=\{1\}$, then there are no non-trivial isocrystals on $X$. The source describes this as a vast strengthening of Gieseker's conjecture and does not state a resolution.