Hi Bradley, and welcome to the BrickWood forums.
If you are looking at the instructions for a BrickWood oven, I think you are confusing the directions for the insulating base with those for the high temperature mortar.
The thing they have in common is that they both use Portland cement as a binding agent. But they are wildly different in purpose.
For the insulating base, you are using Perlite mixed with Portland cement as a thermal break to keep your hearth slab from drawing away heat from the oven door. It never sees direct heat, not does it come in contact with the firebrick in the hearth.
For the oven frame and arch, you need a high temperature mortar. Portland cement is one ingredient in that blend as well, but you also have silica sand, hydrated lime, and most importantly fireclay in the mix. There is NO Vermiculite or Perlite in the mortar. The combined ingredients give the mortar mix high temperature resistance and structural durability that would never be achieved by Portland cement on its own.
You can also use a DRY high temperature mortar such as HarbisonWalker KS4 (the route I chose), but it will add significantly to the finished cost of your build.
I highly recommend the simple mix for the insulated base, and it’s your call (and wallet) as to whether you want to mix your own high temp mortar or buy it as a dry mix. You MUST NOT use a wet premixed product in a pail; it is not suitable for this use.
Please keep posting questions after you’ve looked over the oven plans again. There are plenty of areas for creativity here, but your refractory materials need to hew to specifications if you want the oven to perform.