Partly plastic. Likely urethane, according to the story. Fibers from the cactus are used to make the rest. I sincerely hope no one here thought that the phone cases were made 100% from plant material, that would be a manufacturing nightmare.
"Rubber" products that we purchase all the time are rarely 100% rubber. Just like iPhone cases that we see for sale made with "rubber" are actually made with rubber as part of a mostly synthetic, petrochemical-based material, which is easier for manufacturing and usability consistency, applying permanent dyes to, molding, etc.
But I haven't seen the MR forums descend into bickering about those not being real rubber. It's pretty amazing - and disappointing - how pedantic some of the discussions have gotten here.
It's an iPhone case made party from plant cellulose, full stop. Let's move on.