Cruiser size, labels, and global cruising missions

How can a roughly cruiser-sized Zumwalt still be called a destroyer?

Because official labels are inconsistent; by size and function it is effectively a cruiser, just as some modern frigates perform historic cruiser duties.

The Zumwalt class is cruiser-sized regardless of its official destroyer label. Naval type names can diverge from a ship's displacement and assigned mission. Historically, frigates were dispatched on independent cruising operations. Type 31 global-presence and patrol duties similarly revive a cruiser function under a frigate name. Some modern frigates also approach the size of older C-class cruisers, so role and scale reveal more than the official label alone.