Rear Shroud Kit MANDATORY? (Quad CB1200 Pellet Insert)

Hi all, I'm new to the forum.

I have a Quadra-Fire CB1200I-B Pellet Insert.

Is the Rear Shroud Kit (and associated vent pieces) discussed in Step 7 of the Owner's Manual a mandatory installation on this unit?
Or is it only for factory-built and zero-clearance installations? This manual is quite ambiguous.

I'm installing this in an old-school masonry firebox/chimney. I was of the understanding that I would just slide it into the firebox and hook up some stainless steel Pellet-Vent flex pipe and away I burn.

By the way, I bought this thing brand new a few years back, and I'm just getting around to installing it

Thanks for the help!