This was my exact intuition. At 450 metric ton, we're three orders of magnitude away from what large container ships can do. It's a nice PoC but this is clearly just PR from DHL.
Air freight is also an odd comparison since it's usually time-sensitive and/or pricey ($100+ per pound).
Smallish container ships are used a lot. Look at traffic from the big ports like Rotterdam to small ports like Orkanger or Kristiansand.
A lot of containers take a small-ship trip after the big-ship one.
It's DHL. If you want your package or pallet shipped to some island it might take a long while until a big container ship makes the next stop there. If it can stop there at all. For a package/freight company, the capability to run ships on your schedule to small harbors is valuable. And a catamaran could be a competitive alternative to other smaller container ships
What I don't understand is that they are talking about running it trans-Atlantic. Taking longer than a normal container ship, while taking less cargo. You save on fuel, but surely the crew costs must be eating up all those savings. And you're not really faster. Unless the plan is to go point-to-point between smaller harbors, making up any lost time by saving on cargo handling time