You're probably right. I just caught a quick glimpse of it. It may have been the lighting they used that made me think it was metallic.
On the question of authenticity: It's a real Marc Jacobs. If for no other reason than Kirsty is friends with Oprah, who is a big fan of Marc Jacobs, who is on her show a few times a year and gives her bags. People that run these circles wouldn't take as well known a bag as an MJ Stam and use a fake or "inspired by" bag and put it on the air.
Overlooking all that, Marc Jacobs could rightly sue the Jenny Craig company for using a fake (illegal) Stam and I don't see him giving consent to use a knock off.