Most all sofa cushions that are removable have zippers. It's easier, cheaper, and faster than tying to sew them shut after they've been stuffed. Worked in the furniture-making biz for a few years. Stuffing cushions sucked. It was made easier with a vacuum and some silicone spray (wrap the foam in light plastic, vacuum shrink it, put the cover on, release the vacuum, then work the foam into the corners and zip 'em up.
Were they talking about the cushions? I’m aware that those typically have zippers, but I thought I read that the money was in the body/frame of the couch, where a zipper would certainly be out-of-place. Maybe I was just thinking the body because if I were going to hide something in a couch, that’s where I’d put it.