Zline is junk. Do a search here, you’ll see.

I have a Zline, and I like it just fine. It came with a flexible pipe, but the installer used rigid instead. The problem I had is the stainless chimney wrap (whatever you call it), was the wrong size, I had to order a separate part to cover the exhaust pipe all the way to the ceiling. So you might want to check that before you even get started with installation.

POS

Flexible duct is typically aluminum and not a good firestop if there is a grease fire. While steel flexible duct exists for other purposes, the corrugations will tend to trap grease, and increase the chance of a duct fire that while not penetrating the duct, could cause charring or burning of nearby structure in thermal contact.

"Rigid" might not be a good descriptor. The rule calls for steel duct of some gauge sufficient for stiffness, not true rigidity.

Doesn’t Zline expose themselves to law suit by shipping their product with flexible duct which is against code and could cause fire or even death

Just follow the instructions in Chapter 15 Exhaust Systems, sections M1503.3 Exhaust discharge and M1503.4 Duct material.

They are clueless chinese importers of aggregated parts and cannot produce an appliance worth buying. Of course they don't care about liability. They can just fold the US import front, and open a new one with a different name.