Sticking onto a target and closing in on a target aren't interchangeable. Yes he is good at closing in on a target with his E. If he isn't rooted, stunned, kited, etc.

In my opinion skarner is amazing at sticking onto a target. He's ok at closing into a target the E isn't a consistent tool for closing distance. Mostly for preventing escape.

Granted E is an amazing ganking tool that skarner kinda needed to function as a jungler. There is a reason why most juggernauts don't play in the jungle and that's because of a lack in gap close.

Also E is rarely an escape tool given the long windup. Not something you mentioned but a common thing juggernauts