Antoine, I stated a concern that these are refractory (cast) bricks made to look like traditional (fired in a kiln) firebricks and that they are not actually firebricks. Your answer doesn't invalidate that concern.
If these bricks are cast and not fired, you're basically talking about the same material as your refractory 'mortar,' so, rather than a traditional firebrick + refractory mortar oven, this is basically all cast refractory (in gray mortar color and brick color). Traditional kilned firebricks, are, by their nature, a more physically and thermally durable product than cast refractory. High quality refractory that's been expertly casted isn't going to be much less durable than firebricks, but it's still not exactly the same thing, and selling a wood fired 'brick' oven that doesn't actually contain kilned bricks may not be fraudulent, but, if this is the case, it would rub me the wrong way.
So, are these traditional kilned firebricks or are they a similar cast refractory material to the rest of the oven, but in a different color?