One has wheels, one doesn't.
Ummm, okay?
The complete original quote is,
Though some regular travelers swear by soft bags — the New York-based stylist Ian Bradley, 38, favors the capacious extra-large L.L. Bean Adventure duffel — most are committed to hard, rolling suitcases. “They’re lighter,” says Hitha Palepu, 39, a New York-based author and entrepreneur who wrote what might be the definitive book on the topic, 2017’s “How to Pack: Travel Smart for Any Trip.” The polycarbonate material used for hard-shell suitcases, she explains, “is about half the weight of the rugged nylon used in soft suitcases.”...
A completely useless paragraph that's not accurate. That LL Bean bag is stated to be 2.8 pounds, while all of the hardcase referenced in the 3 bullet points are stated to be:
Rimowa Cabin model is 9.5lbs
Rimowa Essiential Lite Cabin is 4.9lbs their lightest advertised bag
Béis The Small Carry-On Roller is 7.16lbs
Away The Bigger Carry-On: Aluminum Edition is 10.6lbs, but it aluminum rather than PC
Away The Bigger Carry-On made of PC is still 7.9lbs
Like I said the ebags Mother Lode Travel Backpack is 3.9lbs and the July bag in the cntravler article linked above, finally gets a hardshell to 3.9lbs. Mass is mass regardless. The paragraph might have been more accurate or effective had It said something like, though some regular travelers swear by soft bags, most are committed to hard, rolling suitcases. They're easier to maneuver, says...
But one is not packing lighter because they're using a hardshell roller bag. You still must account for bag's mass and pack less of your own mass.