The destroyer already has 80 cells carrying a mix of Tomahawk, Standard, and ASROC missiles. What does it need this for?
Correct, the author gets some facts wrong. The system is the Multiple Launch Rocket System (MLRS). It was originally mounted on a tracked vehicle. The HIMARS (High Mobility Artillery Rocket System) is merely the wheeled version.
There are a variety of munitions for these systems. None of which have been tested against moving ships nor for mounting and use on ships. Salt water is a bear on electronic systems, so you would have to test them out.
All to mount a system that duplicates capabilities found in current naval systems.
The Zumwalts need a good cannon to both hit and kill other ships and boats (fast attack craft, missile boats, suicide boats, etc...) and strike land targets with 24/7 all weather fires. They might be able to adapt low velocity Army 155 howitzer rounds such as Excalibur for use in the current high velocity cannon.
The MLRS is much cheaper per round than a Tomahawk.
It does have existing missile cells. They have their role. But the Zumwalt was “sold” as a land attack specialist with special long-range guns that could shell a target area on land in preparation for a Marine landing, or similar event.
If it effectively has no guns, then it has lost it’s primary mission. It’s not “useless” (because it has 80 missile cells) but it is somewhat pointless.